Wednesday
July 23, 2014

Posts by Reiny


Total # Posts: 26,857

Grade 12 Calculus
I hope your function looks something like this: R(x) = (5000 - 100x)(30 + x) = 150000+ 5000x - 3000x - 100x^2 = -100x^2 + 2000x + 150000 this is a standard parabola opening dowwards , so it will have a maximum the x of the vertex is -b/(2a) = -2000/-200 = 10 So there should be...

Trigonometry
if sinx = -2/√5 in III y = -2, r = √5 , by Pythagoras, x^2 + 4 = 5 x = -1 in III cosx = -1/√5 tanx = sinx/cosx = (-2/√)/(-1/√5) = 2 sin 2x = 2sinxcosx = 2(-2/√5)(-1/√5) = 4/5 cos 2x = cos^2 x - sin^2 x = 1/5 - 4/5 = -3/5 tan 2x = 2tanx...

math
spent on perches --- x litter -------------- x -326 litter and perches combined = 2x - 326 nesting = 2(2x-326) 2x-326 + 2(2x - 326) = 4050 should be relatively easy to solve for x I get x = 838

Algebra 2
help with what? you didn't ask a question. your function simplifies to ... d(x) = 54x^10/(24x^8) = 4x^2 / 9 surely that wasn't your question, was it ?

MATH
so 2 + 4 + 8 + 16 + ... = 178 sum(n) = a(r^n - 1)/(r-1) 2(2^n - 1)/1 = 178 2^n - 1 = 89 2^n = 90 I know 2^6 = 64 2^7 = 128 2^8 = 256 So I agree with your answer , at the end of 7 you only have 128 but you need 178 so it must be the 8th month how did they come up with 10? perh...

Math
first tell me what you got for your inverse matrix

Geometry B
x^2 + y^2 − 2x + 2y − 1 = 0 , notice how we indicate exponents complete the square x^2 + y^2 − 2x + 2y = 1 x^2 - 2x + 1 + y^2 + 2y + 1 = 1+1+1 (x-1)^2 + (y+1)^2 = 3

math
http://www.wolframalpha.com/input/?i=y+%3D+1%2F%283-x%29 asymptote at x = 3

discrete mths
(white, red) ----> 12 13 14 15 16 (5 of them ----> 23 24 25 26 (4 of them) ----> 34 35 36 (3 of them) ----> 45 46 (2 of them) ----> 56 just one so 5+4+3+2+1 = 15 prob(your event) = 15/36

algebra 2
complete the square ... y = -4(x^2 - (3/4)x .....) - 6 , (we usually don't say +-6 to adding a negative 6 ) = -4(x^2 - (3/4)x + 9/64 - 9/64) - 6 , ( take 1/2 of 3/4 to get 3/8, now square that ----> 9/64 = -4( (x - 3/8)^2 - 9/64) - 6 = -4(x-3/8)^ + 9/16 - 6 = -4(x - 3/8...

Trigonometry
http://www.wolframalpha.com/input/?i=y%3D2sin%28x-2pi%2F3%29 for x-intercepts, y = 0 2sin(x-2pi/3) = 0 sin(x-2pi/3)=0 but sin 0 = 0 and sin π =0 and sin 2π = 0 so x - 2π/3 = 0 and x -2π/3 = π and x - 2π/3 = 2π x = 2π/3 or x = π + 2&...

math
amount in B ---- x amount in A -----x - 8000 .04x + .08(x-8000) = 1040 times 100 4x + 8(x-8000) = 104000 I am sure you can finish it

Algebra
-5 + 2x^2 = -6x 2x^2 + 6x - 5 = 0 x = (-6 ± √(36 - 4(2)(-5))/4 = (-6 ± √76)/4 = (-6 ± 2√19)/4 = (-3 ± √19)/2

Implicity
1 + 4x dy/dx + 4y (1) = 2y dy/dx 1 + 4y = 2y dy/dx - 4x dy/dx dy/dx(2y - 4x) = 1 + 4y dy/dx = (1+4y)/(2y-4x)

Calculus A
I will assume you meant: h(x) = x^2 + 5x + 4/(x-1) h ' (x) = 2x + 5 - 4/(x-1)^2 = 0 for max/min 2x + 5 = 4/(x-1)^2 (2x+5)(x^2 - 2x + 1) = 4 2x^3 - 4x^2 + 2x + 5x^2 - 10x + 5 = 4 2x^3 + x^2 - 8x + 1 = 0 hard to solve, Wolfram has this http://www.wolframalpha.com/input/?i=2x...

math
after 1st round 7 people receive email after 2nd round 7^2 or 49 receive email after 3rd round 6^3 or 343 receive email after 4th round 7^4 or 2401 receive email after 5th round ...........16807 receive email number receiving email in 5th round = 7^5

math
correct, so .....

math
what is √(300^2 + 50^2 ) ?

Maths
in groups of 4 with 2 left over: 6 10 14 .... 94 98 102 106 110 114 in groups of 5 with 1 left over : 6 11 16 21 .....96 101 106 111 116 .... looks like 106 or 4a + 2 = 5b + 1 4a = 5b - 1 a = (5b-1)/4 but 5b+1 > 100 5b > 99 b > 99/5 or 20, 21, ... let b = 21, then a= ...

Algebra 1 Answer Evaluation
check by expanding your answer, you will see that the first one is wrong you probably meant a^3 - 5a^2 + 2a -10 , says by grouping, so do that ... = a^2(a-5) + 2(a-5) = (a-5)(a^2 + 2)

Algebra
17*w*(2w+4) = 2720 2w^2 + 4w = 160 w^2 + 2w + 1 = 80+1 , I completed the square (w+1)^2 = 81 w+1 = ± √81 w = -1 ± 9 = 8 or some negatiave, which I will reject the width = 8 inches

Math
general equation of a conic: Ax^2 + Bxy + Cy^2 + Dx + Ey + F = 0 we can eliminate the xy term by finding the angle of rotation Ø , so that tan (2Ø) = B/(A-C) and replacing the x and y with a new X and Y, so that x = XcosØ - YsinØ and y = XsinØ...

Algebra
If x=-4 and x = -1 are zeros to one or more of the given functions, then dividing them by both x+1 and x+4 should yield a remainder of zero. I divided x+1 into each of the given functions (I used synthetic division), each yielded a remainder. So x+1 did not divide into any of ...

good catch , Calcules
yup, looked over my work and found an error in my SA substitution. SA = 2(4xy) +2(x)(4x) + 2(xy) .... = 10x(231/x^2) + 8x^2 ---->10x(231/(4x^2)) + 8x^2 to produce your answers

Calcules
let the height be x then the width is 4x , (the height is 1/4 of the width, this way I have no fractions ) let the length be y volume = (x)(4x)(y) = 4x^2 y 4x^2 y = 231 y = 231/(4x^2) SA = 2(4xy) +2(x)(4x) + 2(xy) = 8xy + 8x^2 + 2xy = 10xy+8x^2 = 10x(231/x^2) + 8x^2 = 2310/x +...

MATH HELP!!
from the 2nd: 2c + 6d = -34 c + 3d = -17 c = -3d-17 plug into the 1st: 3(-3d-17) - 7d = -3 -9d - 51 - 7d = -3 -16d = 48 d = -3 then c = -3(-3) - 17 = -8 c = -8 , d = -3

Algebra II
double the 2nd c - 6d = 16 add to the first 3c = 30 c = 10 back into the 1st 20 + 6d = 14 6d = -6 d = -1 c = 10 , d = -1

algebra 2
what is 3500(1.025) ?

trig
sin(anything) lies between -1 and +1 so -18sin(71π t) lies between -18 and +18 then p lies between 83-18 and 83+18 or between 65 and 101 period of your function = 2π/(71π) = 2/71 min or appr 1.69 seconds 1.69 seconds = 1 beat 1 second = 1/1.69 beats 60 seconds =...

maths
distance along flight path = (3/4)(220) = 165 km measured horizontally: cos 25° = x/16 x = 165cos28 = 49.54 km

Math
Just noticed that Steve answered the same question when you reposted it just a few entries above this Please avoid reposting the same question, especially when it still shows near the top of the list of questions like it did here. It avoids unnecessary duplication of solutions.

Math
vector ab = (-3,4,1) vector dc = (3, -4, -1) = vector cd since their magnitude is the same, and they simply have opposite direction they must be equal and parallel So the figure is a parallogram vector cb = (0,-2,6) area = cross product of ab and cb = (-3,4,1) x (0,-2,6) = (26...

Math
loan at 2.8% --- $ x loan at 4.5 % ---- $ 31000-x solve for x .... .028x + .045(31000-x = 1024.40 times 1000 28x + 45(31000-x) = 1024400 etc

Algebra
Just noticed a typo (sticking 3 key) <taking the first 3 I got: a = 5.5 , b=-12.5 , c = 12> should say: taking the first 3 I got: a = 53.5 , b=-12.5 , c = 12 btw , I did not show how I solved those 3 equations in 3 unknowns, I used a webpage for that http://www.1728.org/...

Algebra
I did the usual analysis and found the first differences to be 145, 250, 358, and 465 and the second differences to be 105, 108, and 107 which appear to be relatively constant at appr 106, so I suspect a quadratic. let N = ad^2 + bd + c, where d is the number of days, and N is...

Math (help)
So the punch contained 1 gal of fruit juice and 1 gal of ginger ale, for a total of 2 gallons percentage of fruitjuice = 1/2 * 100% = 50% I have no clue how they get 25%, did you check the wording of the problem?

Math
66 2/3 of 39 = (200/3)(39) = 7800/3 = 2600

Math
If you have consecutive values of x, like in both your cases, in general if the first differences in the y's are constant, you have a linear function if the 2nd differences in the y's are constant, you have a quadratic function, (as your case above) if the 3rd differen...

Math
I took first differences and got 2.8 , 8.4 , 14, and 19.6 second differences yielded: 5.6 , 5.6 , 5.6 which tells me that the relationship is a quadratic function. so let y = ax^2 + bx + c, giving us 0 = c 2.8 = a + b + c 11.2 = 4a + 2b + c we know c = 0 so if we double the 2n...

math
sinhx = (e^x - e^-x)/2 and coshx = (e^x + e^-x)/ , by definition. then: 5coshx + 3sinhx = 4 5(e^x + e^-x)/2 + 3(e^x - e^-x)/2 = 4 5e^x + 5e^-x + 3e^x - 3e^-x = 8 8e^x + 2e^-x = 8 times e^x 8 (e^x)^2 + 2 = 8e^x a quadratic, let a = e^x 8a^2 - 8a + 2 = 0 4a^2 - 4a + 1 = 0 (2a - ...

maths
The correct answer to your question is: It is a quadratic equation. If you want the solution ... it factors: (3x-1)(3x + 2) = 0 x = 1/3 or x = -2/3

precal
from sin 2x = 1/2 2x = π/6 or 2x = π-π/6 = 5π/6 x = π/12 or 5π/12 but the period of sin 2x is π so we get 2 more solutions: x = π/12 + π = 13π/12 or x = 5π/12 + π = 17π/12 in [ 0 , 2π] x = π/12 , 5π...

Math - PreCalc (12th Grade)
First of all , I am sure you meant: s(t) = 2t^2 + 3t s(2) = 2(4) + 3(2) = 14 s(4) = 2(16) + 3(4) = 44 total distance = 44-14 = 30 m total time = 4-2 seconds = 2 seconds average velocity = total distance / total time = 30 m/2 seconds = 15 m/s

Math - PreCalc (12th Grade)
very similar to http://www.jiskha.com/display.cgi?id=1395410253

Math - PreCalc (12th Grade)
Use you "sense" of numbers to see what happens to the fractions as n gets larger. (I think your last term should read 15/(3n^2) or else it becomes large as n gets large) e.g. look at 5/n suppose n = 100,000, then 5/n = .00005 , which is close to zero suppose n = 1,00...

Math and Music
Perhaps you will find it fascinating to see the relationshipe between music and the Fibonacci Numbers and the Golden Ratio ( google Fibonacci numbers to see thousands of webpages, here is probably one of the best: http://www.maths.surrey.ac.uk/hosted-sites/R.Knott/Fibonacci/fi...

arithmetic
interest = 2000(1.0155)^20 - 2000 = 720.37 or "appr" $ 720

Math - PreCalc (12th Grade)
spread = 5-2 = 3 if cut into n pieces, each base = 3/n endpoint of 1st rectangle = 2 + 3/n endpoint of 2nd rectangle = 2 + 2(3/n) ... endpoint of kth rectangle = 2 + k(3/n) = 2 + 3k/n f(2+3k/n) = 2(2 + 3k/n) + 1 = 5 +6k/2 I see you have two other questions above that follow th...

MATHS
Steve's answer and mine seem to contradict each other, but we defined x differently. Steve defined x as his usual time, so the answer of 7.5 is correct for his usual time I defined x to be the usual speed, and got x = 2/15 so to go 1 km would take 1/(2/15) = 15/2 = 7.5 , t...

MATHS
usual speed --- x km/h time at usual speed to go 1 km = 1/x hrs time at 3/4 his normal speed = 1/(3x/4) = 4/(3x) 4/(3x) - 1/x = 2.5 times 3x 4 - 3 = 7.5x 7.5x = 1 x = 1/7.5 = 2/15 km/h

Trigonometry
r^2= 7^2 + (-3)^2 = 58 r = √58 since (7,-3) is in IV, cosØ = 3/√58 secØ = √58/3 cotØ = -7/3

CORE MATHS
Unless the x term had a coeffecient of zero, then of course you wouldn't count it, so 5a + 10b = 0 ---> a = -2b and 45/2 a^2 + 45b^2 = 90 (45/2)(4b^2 + 45b^2 = 90 90b^2 + 45b^2 = 90 135b^2 = 90 b^2 = 90/135 = 2/3 b = ±V2/√3 if b = +√2/√2 then a ...

CORE MATHS
(1+ax/2)^10 + (1+bx)^10 = (1 + 10(ax/2) + (10)(9)/2 (ax/2)^2 + ... ) + (1 + 10bx + (10)(9)/2 (bx)^2 + ...) = 2 + (5ax + 10bx) + (45/4) a^2 x^2 + 45b x^2 + .. = 2 + (5a+10b)x + (45/2 a^2 + 45b^2)x^2 + ... first term matches your given, but the 2nd only contains first degree x ,...

math
roughly plot the points to see that you have a rectangle of base = 30 height = 2 area = 2x30 = 60 square units

math
term 11 = a+10d = 8+10d term 7 = a + 6d = 8+6d (8+10d)/(8+6d) = 5/8 64 + 80d = 40 + 30d 50d = -24 d = -24/50 = -.48 check: term 11 = 8 + 10(-.48) = 3.2 term 7 = 8+6(-.48) = 5.12 5.12 : 3.2 = 512 :320 = 8 : 5

Physics question - I NEED HELP PLEASE
need rules of physics here. volume of water if trough is full = (1/2)(3)(5)(8) = 60 cubic metres from here on it is physics

math
boys --- 4x girls ---- 5x 4x+5x = 54 take over....

Math
distance at 15 km/h --- x distance at 18 km/h ---- 136-x x/15 + (136-x)/18 = 8 multimply each term by 90 6x + 5(136-x) = 720 6x + 680 - 5x = 720 x = 40 distance at 15 km/h is 40 km distance at 18 km/h is 136-40 or 96 km check: 40/15 = 2 2/3 hrs 96/18 = 5 1/3 hrs , for a total ...

math
(1/8) of what number is 3 ?

derivatives
yup! sloppiness on my part

derivatives
I assume that you want the derivative, since you titles your post that way. if y = x^2cosx-2xsinx-2cosx dy/dx = (x^2)(-sinx) + 2x cosx - 2x(-cosx) - 2sinx + 2sinx = -x^2( sinx) + 4x( cosx) etc if needed

Not so fast .....
Since each flower needs 1 square foot, the flowers planted along the circumference will not have a complete square foot. To illustrate, take a grid paper, and draw a circle with radius 7. we will need x^2 + y^2 = 49 you only have to look at 1/4 of the circle and look at the nu...

Calculus Help
y = (e^-x) (cosx)^2 (x^2 + x + 1) take ln of both sides ln y = ln e^-x + ln (cosx)^2 + ln(x^2 + x + 1) = -x + 2 ln(cosx) + ln(x^2 + x + 1) now differentiate y' / y = -1 + 2(-sinx/cosx) + (2x+1)/(x^2 + x + 1) = -1 - 2tanx + (2x+1)/(x^2 + x + 1) y' = y(-1 - 2tanx + (2x+1...

Math
for any quadratic function f(x) = ax^2 + bx + c the x of the vertex is -b/(2a) sub that x value into the function to get the y of the vertex, so n(x) = -.3x^2 + 9x + 15 x of vertex is -9/(2(-.3)) = 15 n(15 = -.3(225) + 9(15) + 15 = 82.5 ticket sales will peak on day 15 and the...

math
TC = 5/9 (TF – 32) TF = 9/5 TC + 32 , converts between TF and TC TF is temperature in archaic Fahrenheit, TC is temperature in Celsius I + PRT, interest = principal x rate x time for simple interest D = R x T , distance = rate x time

Math
yes , the x-intercepts are 0 and 4, respectively confused about your second reply, the vertices are indeed (0,0) and (0,4) <<I mean 0 for -2x^2. Sorry about the 4, I didn't mean to put that. >> , don't even know what that is supposed to say. see, http://www...

Math
You talk about equations, but show no equation. You must have meant the equations to be y = -2x^2 and y = -2x^2 + 4 You are asking for the axes of symmetry but instead state the vertices. You were correct that the vertices are (0,0) and (0,4) respectively. so the axes of symme...

Math
Did you not notice that for every increase of 20 in the x's there is a decrease of 20 in the y's so the slope is (45-65)/(30-10) = -1 so y = -x + b using x=10, y = 65 65 = -10 + b b = 75 y = -x + 75

trigonometry
the formula you want is cos a + cos b= 2cos((a+b)/2) * cos((a-b)/2) so cos 5w + cos 2W = 2cos(7w/2) * cos(3W/2)

Algebra
I did this one last year http://www.jiskha.com/display.cgi?id=1362104847

algebra 1
first even : x 2nd even : x+2 now do it like the others

PreCalc
Andy, I have done quite a few for you now. Give this one a try, let me know what you get.

uninhibited growth
done, look back to your earlier post of this

algebra 1
done , look back

interest
we don't really need the amount at first rate: 2 = 1 e^(.05t) ln2 = .05t t = 13.86 ---> the doubling time at 2nd rate: 2 = 1 e^(.1t) you finish it.

algebra 1
1st number ---> x 2nd number -- > x+1 3rd number --> x+2 x+x+1+x+2 = -204 3x = -207 x = -69 largest is -67 or let the 3 numbers be (x-1) , x , (x+1) x-1 + x + x+1 = -204 3x = -204 x = -68 largest number is -68+1 = -67 notice that in my second solution, the equation wa...

algebra 1
what does (2/9)x = 24 sound like to you?

algebra 1
looks like (3/8)b = 14

Math
The ones digit is 5 less than 10. ---> xx5 The tens digit is 2 more than the ones digit -- x75 it must be less than 200 so it is either 175 or 75

Precalculus
I have now answered several question for you of this type. This is the easiest of the bunch Your turn ! let me know what you got

PreCalculus
amount = 100 (1/2)^(t/5730) b) 59 = 100 (.5)^(t/5730) .59 = .5^(t/5730) ln .59 = (t/5730) ln (.5) t/5730 = ln .59/ln .5 = .76121314 t = 4361.75 yrs

Uninhibited Growth
time from 1992 to 2010 = 18 years Denmark: pop = 5.2 e(18(.001)) = appr 5.29 million Chad: pop = 5.2 e(18(.025)) = 8.2 million when is 10 = 5.2 e^(.025t) 1.923077 = e^ .025t .025t = ln 1.923077 t = 26.16 years or in appr 26 years or in 2018 (in 2010 it was 12.7 million)

Precalculus
let t=0 correspond with the year 1850 then 1900 ---> t = 50 then 1950 ---> t = 100 population = a( e^(kt) ) , where a is the initial population and t is the number of years since 1850 23,191,876 = a e^0 a = 23,191,876 then: 62,947,714 = 23,181,876 e^(50k) 2.715384812 = e...

math
what is 123(81) - 492(9) + 813 ?

AP Calc
let's eliminate the parameter t from x = e^t lnx = t then y = (lnx)^2 - 3 to be concave up, the second derivative must be positive y' = 2(lnx)(1/x) y'' = 2lnx (-1/x^2) + (1/x)(1/x) = (-2lnx + 1)/x^2 x^2 is always positive so -2lnx + 1 > 0 lnx < -1/2 ----&...

AP Calc
cos y = x - y -sin y dy/dx = 1 - dy/dx dy/dx - siny dy/dx = 1 dy/dx(1 - siny) = 1 dy/dx = 1/(1 - siny) to have a vertical tangent, the slope of that tangent must be undefined, that is, 1 - siny = 0 sin y = 1 y = π/2 or y = -3π/2 or 5π/2 if y = π/2 , then co...

College Algebra
by intuition, it must be 8 oz or the amount of salt must stay the same, so .02(16) = .04(16-x) .32 = .64 - .04x .04x = -.32 x = 8

Trigg
y=1000 sin (2t) +400 sin(anything) lies between -1 and +1 so the smallest value of sin(2t) is -1 so the smallest value of y = 1000(-1) + 400 = -600 But, it would make not make any sense to have a negative population, so my answer would be zero

Algerbra
I will assume I am looking at two different ones 1. -1< 3+2x<11 2. 1 – x >-3 or 5x-1>19 #1. -1< 3+2x<11 subtract 3 -4 ≤ 2x < 8 divide by 2 -2 < x < 4 all real numbers between -2 and 4 #2 1 – x >-3 or 5x-1>19 -x > -4 or 5x >...

math
The use of this page is more efficient and more accurate than using a standard normal distribution table, http://davidmlane.com/hyperstat/z_table.html for the prob(between 170 and 180) is gives .4772 for the prob( > 178) it gives .0548 multiply each of these by the 80 peopl...

French school
This person has listed the number of school days of several countries http://norberthaupt.com/2012/04/20/school-days-around-the-world/ Not all countries are listed of course, e.g. he did not list my Canada, but I know that in Ontario the number of days is 195 on average.

Math 134
Don't try to "create" fractions in this format, the spacing comes out all wrong. did you mean: 2x/(x+3) + 9/(x-3) - 36/(x^2 - 9) ?? if so, notice that x^2-9 = (x+3)(x-3) , so your LCD is (x+3)(x-3) give it a try

Math 134
my goodness! I answered this same question for you earlier. All you have to do here is change the numbers. http://www.jiskha.com/display.cgi?id=1395018684

Alegbra
if those negatives are supposed to be exponents, write them this way: 6.3 x 10^-7 3.25 x 10^-12 both expressions are in scientific notation

Alegbra
I suspect a typo

math
ummh, that would be (0,0) You should know the properties of this , the simplest form of the equation of a circle.

Math 117
done, check back on your previous post of this

Math 117
I interpret (fg)(x) as (-6x-1)(4x^2) = -24x^3 - 4x^2

Math 117
I bet you mean: f(x) = (x+10)^2 new equation: g(x) = (x+10 - 12)^2 = (x-2)^2

math
radius of each bubble = .005 m or (1/2) cm volume of each bubble = (4/3)π(1/2)^3 cm^3 so 1 million of them = 1000000(4/3)π(1/2)^3 = appr 523598.78 cm^3 = appr .5236 m^3 a bit over 1/2 cubic metre

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