Newest questions and responses by Reiny
  1. @Peter, earlier question

    Peter, I found your question interesting and have been messing with it for a while I finally made up a short computer program using a silly old

    asked on March 19, 2019
  2. @Anna, stop it

    are you even looking at some of the replies? see:

    asked on January 28, 2019
  3. @ Steve or Damon

    take a look at this question Am I guilty of overkill or over-thinking this ? Surely there has to be an easier way.

    asked on October 29, 2015
  4. For Hannah

    Re Sorry to have caused confusion for you Hannah. In your original post, you had 3 different problems, the first two were "volume" questions. When I did the third one, which was simply finding an area, I

    asked on May 22, 2013
  5. For Anubhav

    re I have a solution, but what a mess. Before I type it all out, let me know if you still need the solution. I certainly don't feel like typing it all out unless you will actually look at it.

    asked on May 16, 2013
  6. Happy π Day to all mathematicians

    Today, March 14, at 1:59 was a very special time.

    asked on March 14, 2013
  7. Damon or Steve or bobpursley or drwls or ...

    any ideas ? , looks way too messy for here

    asked on February 17, 2013
  8. To Jessica

    I have now corrected two of your replies in which you gave incorrect solutions. Unless you know what you are doing, please don't answer any questions dealing with topics not familiar to you . I know everybody makes mistakes, I make my share, but those are

    asked on January 31, 2013
  9. for Knights

    have been looking at your post ... interesting question... let the altitude of 12 hit base a let the altitude of 14 hit base b then (1/2)(12)a = (1/2)(14)b 6a = 7b a = 7b/6 let the third side be c and its

    asked on January 1, 2013
  10. For MathMate or bobpursley - check this

    Can somebody take a look at this?

    asked on October 13, 2011
  11. For Lia - question from this morning

    I took some time to work out your rather "nasty" Calculus question. Take a look at it and see if it matches your expected result.

    asked on March 23, 2011
  12. MathMate - look at this

    perhaps you can shed some light on this I seem to have a "mental block" here.

    asked on December 13, 2010
  13. For Carry, Alissa,Courtney, Max

    Please do not switch names, it causes unnecessary and duplicate replies from tudors.

    asked on November 14, 2010
  14. Happy Thanksgiving Canada

    Happy Thanksgiving to my fellow Canadian tutors and to the Canadian students at Jiskha.

    asked on October 11, 2010
  15. For Lily

    Please do not append a new question at the end of somebody else posting. Your question will likely be passed over, since most tutors will not look at a posting if they see that it has been answered.

    asked on July 7, 2010
  16. To Math Students, re √ sign

    Several student tonight have asked me how I create the √ symbol to create √ symbol on a PC, hold down the Alt key, and at the same time press 251 on the number pad, then release the Alt key on Mac, hold down the Option key, and press V, release the

    asked on June 9, 2010
  17. ATTENTION Ms Sue

    spamming here

    asked on May 27, 2010
  18. Ms Sue -- Welcome back

    hope you had a great trip

    asked on May 12, 2010

    You now have 6 different posts going for the same "cake question" I am not going to keep going back to see which one you replied to. Stay with one posting please. I just notice one way down the list where you did part a) Calculate the area on top of the

    asked on April 22, 2010
  20. For <b>marie flore joseph</b>

    Please do not add your new question as a response to somebody else's post. click on "Post a New Question" above to post your problem. Here is your question : Math - marie flore joseph, Monday, April 5, 2010 at 10:37pm the difference of the squares of two

    asked on April 5, 2010
  21. Happy Pi Day

    Happy Pi Day to all math "nerds" Even Google changed their log today.

    asked on March 14, 2010
  22. Message for KMTRENd

    One would have thought that the least you could have done was give an acknowledgment for a most extraordinary reply you got to your question by tchrwill

    asked on February 3, 2010
  23. To All Students & Tutors

    I will not be able to answer any questions on this board for the next 8 days or so. I will be warming my weary bones down south off one of the islands of Belize for a while. Great time to get away as the temperature up my way is currently at -12 C. Reiny

    asked on January 6, 2010
  24. For Haley

    Even though you mean well, your last three responses have been either incomplete or incorrect. The one to is totally off the mark. Please make sure you are familiar with the topic before replying.

    asked on November 14, 2009
  25. For MUFFY

    You stated on Saturday that you understood my solution as far as part a) and b) were, so let me just recap what we did. (I will not repeat the actual question) My Solution: Suppose we label the point of contact P(x,y). I bet P is in the first quadrant. but

    asked on October 5, 2009
  26. Nice unanswered question by Maggie

    "100 bushels of corn are divided among 100 men, women, and children. Men receive 3 bushels each, women 2 bushels and children 1/2 bushel each. How can the bushels be distributed? is there more than one solution? if so, find the other solutions." let the

    asked on September 16, 2009
  27. Attention : Caroline

    check your post from

    asked on April 8, 2009
  28. All Done

    Ok, logging off, night all.

    asked on February 24, 2009
  29. Message to Students re Math

    Just thought I would let you young'nes know that being a Mathematician has just been chosen as the most rewarding profession. Now I know why I have been so happy all my life, lol So keep

    asked on January 8, 2009
  30. Attention : H Masri

    re your post from late last night

    asked on November 6, 2008
  31. Attention: Courtney's Dad

    asked on October 26, 2008
  32. Reiny to Gina (logic)

    I saw your reply re my late night solution to your pumpkin question I did indeed make two typing errors, which even though they do not change the actual answer, nevertheless would have caused confusion. I

    asked on October 17, 2008
  33. Is Form Going to the dogs?

    sorry, just couldn't resist that.

    asked on October 6, 2008
  34. For George

    At 11:37 and at 12:24 you posted the same question the earlier one was answered correctly by drwls by accepting your question the way you typed it. my solution is based on the assumption that your denominator was e^(t-7), an invalid assumption.

    asked on September 11, 2008
  35. Algebra

    3(2+3-2(4-5)2-1)-3(2+4(4-2)-8+3(2-5)+1-2)1-3)+4-5 rather silly question, and some of it makes no sense eg look at the first part ......-2(4-5)2-1 does this end up -2(-1)(2) ? or did you forget an operator before the last 2? there is a similar discrepancy

    asked on August 21, 2007
  36. math.algebra

    other than factoring out the 9 there is really no simplification that can be made here 9sqrt6 - 9sqrt2 =9(√6 - √2) can someone help me simplify: 9sqrt6 - 9sqrt2

    asked on July 29, 2007
  37. college Algebra

    This is "college" math? Anyway... let the smaller number be x, and the larger number be x+8 then x + x+8 < 12 this is easy to solve, let me know what you got. the sum of two numbers is less than 12. The second number is 8 more than the first.what are the

    asked on July 25, 2007
  38. maths

    Set it up this way: Let the number of students be x let the number of rows be y then the number of students per row is x/y This results in 2 equations from your data: (x/y+1)(y-2) = x and (x/y-1)(y+3 = x Solve these two, gets a bit messy at first but it

    asked on July 15, 2007
  39. algebra

    These are very easy to set up with a "chart" ---------Distance-----Rate---Time (D=RxT) slow way --6x----------x-------6 fast way - 5(x+7)-----x+7------5 so 6x = 5(x+7) 6x=5x+35+ x=35 The speed on the slower trip was 35 mph, and on the return trip 42 mph A

    asked on June 17, 2007
  40. math

    break it down into cases at most one day of rain in 3 days to me means we could have: norain norain norain ----> (1/3)^3 = 1/27 rain norain norain -------> 2/3(1/3^2 = 2/27 norain rain norain -------> same as above = 2/27 norain norain rain -------> same

    asked on May 17, 2007
  41. I can write it instead

    Leo, a similar prediction is often made about arithmetic since the invention of the calculator. in one of Isaac Asimov writings they discover a man who is actually able to perform an operation like 368 x 4821 with something called a pencil and paper. My

    asked on May 15, 2007
  42. math

    You did not watch your units, you are mixing inches and feet Your equation should have been 1/2 = 1/di + 1/120 (I changed 10 feet to 120 in) solve this for di and you get 2.03 inches. Please help me I am so confused I think I am losing my mind. The focal

    asked on May 9, 2007
  43. Vectors-another angle confusion

    On an x-y grid, label them N-S, E-W, draw a line from the origin to the right and label it 425 (you are going East) From the end of that line draw a vertical line upwards of 40 (you are going North) Use an approximate scale, realizing that 425 is just

    asked on April 20, 2007
  44. Vectors

    I hope that the diagram I have drawn for myself matches the one in your question. I have a triangle with sides 20 and 10 and an angle of 120º between them By the cosine law I found the third side to be 26.4575. You said you found the maginitude, I hope we

    asked on April 20, 2007
  45. Trig

    I don't know the physics behind Snell's Law, but I can do the math Snell's Law says sin(angle of incidence)/sin(angle of reflection) = refractive index since you know the two angles... sin45/sin27 = refractive index =1.5575365 so for an angle of refection

    asked on April 19, 2007
  46. help, bobpursely

    Please take a look at DEVON's trig equation at 9:14 I must be missing something obvious. If the second had been 3sin 2x it would be a straight forward question. The way it stands I was able to "reduce" it to the equation 8sin^4 x - 8sin^2 x + 3sin x + 2 =

    asked on April 16, 2007
  47. math

    how about (5-3)x2 + 8(5-2) ? To me these are really just "puzzles" and do not actually require much mathematical ability. The best and often only way is to by 'trial and error'. what is the rule when grouping numbers? example I want the answer to be 28. 5

    asked on March 28, 2007
  48. algebra

    I trust you know about y = mx + b Realize that when you are given the x or y intercepts, you are really just given two points, in your case the points (4,0) and (0,-2) So it is easy to find the slope, (-2-0)/(0-4) = 1/2 Take it from there Determine the

    asked on March 28, 2007
  49. algrbra

    Complimentary angles add up to 90º So translate the English into Math. if you let the unknown angle be x then "the measure of the compliment of the angle" -----> 90-x "10 degrees less than three times the measure of the angle" ----> 3x - 10 so....... 90-x

    asked on March 28, 2007
  50. principles of finances

    according to my interpretation of the question in October they would collect: the 10% of the August sales = .10($10,000) the 20% of the Sept sales = .20($20,000) and the 70% of the Oct sales = .70($30,000) Do the arithmetic and add them up A project has an

    asked on March 28, 2007
  51. calc

    I assume the poster was trying to type "oblique asymptote" from your answer of 3x+2 + 7/(x-1) we can conclude that there is an oblique linear asymptote of y = 3x + 2, and a vertical asymptote at x = 1 for the original function given. how do u find obblique

    asked on March 26, 2007
  52. Monitors of Board

    Is there any way that the postings could be numbered? The post below this one by TONY obviously refers to a previous posting of his 8 steps below. I know the URL has a number behind each particular post, and each post can be identified by a title and

    asked on March 20, 2007
  53. factoring

    let f(m)=m^3 + 3m^2 + 3m + 1 notice f(-1) = 0, so m+1 is a factor. Using division, the answer is m^2 + 2m + 1 which factors once more to (m+1)(m+1) so you have (m+1)^3 = 0 and then m = -1 Are you familiar with Pascal's triangle? Did you notice the pattern

    asked on March 11, 2007
  54. math

    There is no function equation for the Fibonacci numbers, you have to use a Recursion formula. F(n) = F(n-1) + F(n-2) for n>2 and F(1)=1, F(2)=1 what is the equation for the fibonacci sequence

    asked on March 11, 2007
  1. Math

    arc = rθ in our case: arc = 7θ d(arc)/dt = 7 dθ/dt 10000 m/hr = 7 m dθ/dt dθ/dt = 10000/7 rad/hr or appr 1428.57 rad/hr = appr 23.8 rad/min or , non-Calculus way circumference = 14π = 43.98 m speed = 10 km/h = 10000/60 m/min = 166.667 m/min time to

    posted on March 23, 2019
  2. Math

    I will assume that your equations are supposed to be: y = (-1/3)x + 3 and y = x + 2 your choice of C of (-1,3) satisfies the second but not the first equation. I see only two that satisfy the second equation only so until you properly type the first

    posted on March 22, 2019
  3. Math

    A's rate = 120 hours/ 4 years = 30 hrs/year B's rate = 1.2(30) or 36 hours/year so after 1.5 years, A had 45 hours and after 1.5 years, B had 54 hours

    posted on March 22, 2019
  4. Math

    Probably, how many dimes and quarters does he have? Quarters --- x dimes ----- 25-x solve for x: 25x + 10(25-x) = 400

    posted on March 22, 2019
  5. math/pre-calc

    First of all, your equation starting with -4.9t^2 + .... would be valid for metres and not for feet, on top of that the +100t would be the correct term for units in feet. All other units are in feet, what a messed-up question. So check on the wording of

    posted on March 22, 2019
  6. math

    He started his collection with 25 cards and adds 10 new cards every month

    posted on March 22, 2019
  7. Maths

    Treat the Father-Mother as one person, so now you are seating only 6. Let's put MF in one of the seats, so with caring about the actual place around the table there would be 6*5! ways to do that. BUT, supposedly there are no distinct seats, so the number

    posted on March 22, 2019
  8. Maths

    Assuming that by "bearing" you are using the notation that North = 0° and clockwise is positive, then you have a triangle ABO, where O is the origin and angle BOA = 116° use the cosine law: AB^2 = 10^2 + 30^2 - 2(10)(30)cos116 find AB then use the

    posted on March 21, 2019
  9. Math

    2π radians every 25 seconds x radians every 60 seconds x/(2π) = 60/25 solve for x

    posted on March 21, 2019
  10. Math

    look at this question from 2007 Yours just has different constants let the first even number be x the the next even number is x+2 1/x - 1/(x+2)

    posted on March 21, 2019
  11. Math

    Bob's speed --- x km/h Al's speed ----- 1.1x km/h Bob's time = 264/x and Al's time = 264/1.1x the difference in their times is 18 min = 18/60 hours 264/x - 264/1.1x = 18/60 = 3/10 multiply each term by 110x , and take over to solve for x comes out nice.

    posted on March 21, 2019
  12. math

    i = .04/4 = .01 n = 4(6) = 24 amount = 500(1.01)^24 = ....

    posted on March 21, 2019
  13. Math

    Let the side parallel to the house-wall by y let the other two sides of the rectangle be x so y + 2x = 2L y = 2L - 2x area = xy = x(2L - 2x) = -2x^2 + 2Lx the maximum area occurs at the vertex of this downward parabola the x of the vertex is -b/(2a) =

    posted on March 21, 2019
  14. Algebra

    fraction = 12/30 = 2/5 or .4 or 40% If "they" don't accept that answer, which Barry Vacker also had, then there is either a misprint in the question, or an error in the answer key.

    posted on March 21, 2019
  15. Calculus

    ∫ e^-x dx from 1 to ∞ = [ -e^-x] from 1 to ∞ = [ -1/e^x] from 1 to ∞ = -1/e^∞ - (-1/e^1) = 0 + 1/e = 1/e confirmation:

    posted on March 21, 2019
  16. Algebra 2

    Since the parabola opens downwards, you said that, there is a maximum point That point is called the vertex, you had it incorrectly at (2,12), should have been (2, 4) you should precede your calculation with statements, such as the x of the vertex =

    posted on March 21, 2019
  17. math

    "4x minus 5y equals 3 line break 3x plus 5y equals 13" How about just typing: 4x - 5y = 3 3x + 5y = 13 Don't the y's have opposite coefficients ? Wouldn't just adding them "eliminate" them? Just for fun, you should try the other two methods and compare the

    posted on March 21, 2019
  18. Math

    "translated left 4 units" mean 4 is subtracted from x : mmmhhh? (x-4) ? "up 1unit" , up refers to the y. So how would you add 1 to y ?

    posted on March 21, 2019
  19. Math

    If they have the same direction, then their magnitude is 9 It is also possible to have a triangle with sides 4,5, and 1 clearly not with sides 11 or 13 so A and B

    posted on March 21, 2019

    Weight = 2.5w + 4, where w is the number of weeks. You better put a domain restriction on w or else you will have a "fat cat".

    posted on March 21, 2019
  21. Geography

    So Greenwich, (not Greenwish), is 6 hours ahead of the town so the town must be at 90° West, or -90° (I am in that time zone)

    posted on March 21, 2019
  22. Math

    thanks, oobleck I should really read the questions more carefully.

    posted on March 21, 2019
  23. Math

    one number --- x the other ----- y x+y=12 -----> x = 12-y x - 1/y = 2/9 12-y - 1/y = 2/9 multiply by 9y 108y - 9y^2 - 9 = 2y 9y^2 - 106y + 9 = 0 solve the quadratic, does not come out nice

    posted on March 21, 2019
  24. calculus

    In your step: [0,π/2]∫7cos(x)-7cos^2(x)dx [0,π/2]∫7cos(x)-7(1-sin^2(x))dx why did you change cos^2 x to (1 - sin^2 x) to integrate sin^2 x is just as difficult as cos^2 x did you mean to use: cos(2x) = 2cos^2 x - 1 ?? then 2cos^2 x = cos(2x) + 1

    posted on March 21, 2019
  25. calculus

    You are right, so just turn the terms around to area =∫ (7cosx - 7cos^2x) dx from 0 to π/2 I should have looked at my own graph a bit closer.

    posted on March 20, 2019
  26. math

    cost1 = .02m + 25 cost2 = .03m + 15 when is cost2 < cost1 ? .03m + 15 < .02m + 25 .01m < 10 m < 1000 , so for any time less than 1000 minutes, the second plan is cheaper

    posted on March 20, 2019
  27. Math

    treat it as two ordered pairs of the form (t, h) they would be (12, 212) and (25,95) find the slope and find the equation using the method you must have learned to do this type of problem.

    posted on March 20, 2019
  28. Algebra

    Since it is parallel, it must differ only in the constant, so let y = -4x + b sub in the given point, find b, and you are done

    posted on March 20, 2019
  29. calculus

    first we need their intersection: 7cos^2(x) = 7cos(x) cos^2 x - cosx = 0 cosx(cosx- 1) cosx = 0 or cosx = 1 x = π/2, or x = 0 ahhh, our boundaries happen to be their intersection. let's look at the graph:

    posted on March 20, 2019
  30. algebra

    translate into math .... 4x - 2 = 5x - 2 now solve it. You should have seen the obvious answer by the "just think about it" theorem.

    posted on March 20, 2019
  31. Math

    r=1+sinθ √(x^2 + y^2) = 1 + y/√(x^2 + y^2) x^2 + y^2 = √(x^2 + y^2) + y confirmation:

    posted on March 20, 2019
  32. Math

    suppose you let x = b^2 then your equation becomes x^2 + 2x - 24 = 0 This factors nicely to (x + 6)(x - 4) = 0 x = -6 or x = 4 then b^2 = -6 or b^2 = 4 let's handle the b^2 = 4 clearly b = ± 2 what about the b^2 = -6 there is no solution in the set of

    posted on March 20, 2019
  33. math

    It is important that when you sub into y=-3x^2 the y value will always be negative. make sure you do (-3)(x^2) , so you square the x first, which will never be negative, and then multiply that by -3. So any point in your list with a positive y would be

    posted on March 20, 2019
  34. Math

    Is it arithmetic? Is 3 - 81 = 1/9 - 3 ? , Nope, so it is not arithmetic Is it geometric? is 3/81 = (1/9) / 3 ? , Yes, so it is geometric to be arithmetic, there has to be a common "difference" between consecutive terms to be geometric, there has to be a

    posted on March 20, 2019
  35. maths

    I will assume you meant: if the line x+y = k is a normal to the hyperbola x^2/9-y^2/4 = 1 then k= your hyperbola is 4x^2 - 9y^2 = 36 8x - 18y dy/dx = 0 dy/dx = 8x/18y = 4x/9y at any point on the hyperbola, the slope of a tangent is 4x/9y so the slope of

    posted on March 20, 2019
  36. Algebra

    i = .031/365 = .... n = 365*3 = ... sub into : amount = 8500(1+i)^n

    posted on March 19, 2019
  37. Math

    well the x went from 4 to 0, so horizontal change is -4 the y went from 6 to 8, so the vertical ......

    posted on March 19, 2019
  38. Algebra

    slope = (-4-2)/(7-4) = ....

    posted on March 19, 2019
  39. precalc

    we know a = 50, and the period is 7 minutes 2π/k = 7 k = 2π/7 then let's start with the basic shape of height = 50sin (2π/7)t , where t is in minutes and height is in metres. but the maximum height is 100 m , so we have to raise our curve 50 m height =

    posted on March 19, 2019
  40. Calculus

    L'Hopital's Rule, not L'Hospital's, is used in the evaluation of limits. Is this a limit question? Here is a nice video by Sal of Khan Academy, with an introduction to it.

    posted on March 19, 2019
  41. Math

    I have no idea what you are even asking. Was the 5 obtained by subbing in certain values of x and y ??

    posted on March 18, 2019
  42. Algebra 2

    You are doing "completing the square" in a much-too complicated way: Consider this after your start: 22= -16t^2 + 40t 16t^2 - 40t = -22 divide by 16 t^2 - (5/2)t = -11/8 t^2 - (5/2)t + 25/16 = -11/8 + 25/16 (t - 5/4)^2 = 3/16 t - 5/4 = ±√3/4 t = 5/4 ±

    posted on March 18, 2019
  43. Algebra 2

    You started off well, but then it made no more sense 22 = -16t^2 + 40t 16t^2 - 40t + 22 = 0 using the formula, since it does not factor ..... t = (40 ± √(1600 - 4(16)(22)) )/32 = (40 ± √192)/32 = (40 ± 8√3)/32 = (5 ± √3)/4 =appr 1.68 or a

    posted on March 18, 2019
  44. Algebra

    Since you are looking at the graph, and we can't see the graph, it is your job to simply look at where the two lines intersect.

    posted on March 18, 2019
  45. Algebra

    2x^2-3 = 3x-1 2x^2 - 3x - 2 = 0 (2x + 1)(x - 2) = 0 x = -1/2 or x = 2

    posted on March 18, 2019