math
posted by lynne .
question is 40x^2 + 23x 30 = 0
reads as 40x squared plus 23x minus 30 equals zero...please help, we have not learned the quadric formula yet so we are finding the numbers that make 30 and add to 23.

math 
Reiny
I use a method that nobody seems to either teach or learn anymore, yet it is the fastest way to find the factors if they exist
I make a pair of two columns, one for the factors of 40, another for the factors of 30
(they line up vertically)
2 4 5 2 3 5 15 10 6
20 10 8 15 10 6 2 3 5
I then form crossproducts with a column pair from the first and a column pair from the second
In this case I am looking for a difference of 23, since the last term is 30
Had it been +30 I would look for a sum of 23
I notice my pairs
5  6
8  5
give the crossproduct difference of 5x5  6x8 = 23
so my factors must be
(5x .....6)(8x .... 5)
A quick mental check gives me
(5x+6)(8x5)
Notice I repeat the factors in one of the groups by flipping the order. Most of the time I don't list all the combinations, often skipping the extreme ones like
1
40
In actual practise it only took me about 30 seconds to find the above 
math 
lynne
so the correct answer is (5x+6)(8x5) ?

math 
Reiny
can be easily checked by expanding it.

math 
lynne
i did and came up with the same. however when i entered it they said it was wrong, s/b X= 5/8, 6/5...not sure how

math 
Reiny
Since the hard part was to factor it, I assumed that you would finish it.
so
(5x+6)(8x5) = 0
x = 6/5 or x = 5/8 
math 
Lynne
i totally dropped the ball in this one...i should have definitely know to find x!!!
Respond to this Question
Similar Questions

Math
I can't for the life of me solve this! 6x^223x+15=0 6x^223x+15 = (a x + b) (c x + d) > a c = 6 (1) a d + b c = 23 (2) b d = 15 (3) You could write down all possibilities for a and c (Eq. 1) and all possibilites for b and d … 
Algebra
almost done with Algebra...help! 2 3x – 6(2x – 0.055) = 10x + 0.515 I don't know what want but assuming your solving for x... 23x12x.33= 10x + 0.515 11x.33= 10x + 0.515 x= 0.845 You need to clarify your post. Is that 23x or … 
math
(X^21)/(x1) = (x+1)(x1)/(x1) = x+1. If you then subtract your original number, x, you are left with 1. The reason this doesn't work with 1 is that you would have 11=0 in the denominator and you cannot divide by zero. This uses … 
Math
One solution of 12x^323x^23x+2=0 is 2. Find all other solutions. I tried rearranging the equation to factor by grouping, but that 23x^2 is bothersome and I can't get the right answers (I know the answers are 2, 1/4, and 1/3). Help? 
math
6x (squared) = 23x+18 help please 
math
Have 2 questions: 1. Factor Trinomial competely x^2 + 8x + 53 (reads as x squared plus 8x plus 53) I am thinking it is prime. 2. Solve 4x^2 + 23x  30=0 (reads 4x squared plus 23x minus 30 equals zero 
Discrete Math
Please help with this problem. Thanks a/ Show that for any real number x, if x>1 then x^4=< 23x^4 + 8x^2 + 4x b/ Show that for any real number x, if x>1 then 23x^4 + 8x^2 + 4x => x^4 c/ Use the big Theta and big … 
Factoring Trinomials
Working on how to factor trinomials, and the method that works best for me is the criss cross method, so Ive sticked with it. But Ive been stuck on one question for the past few minutes, and don't know what I've been doing wrong. 4x^2+23x+15 … 
Math
Find all the zeroes of the poly 4x cube minus 20x cube plus 23x square plus 5x minus 6 if two of itz zeroes are 2 and 3 
Calculus AB
Which of the following functions is continuous at x = 3?