posted by lynne on .
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.
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 cross-products 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 cross-product difference of 5x5 - 6x8 = -23
so my factors must be
(5x .....6)(8x .... 5)
A quick mental check gives me
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
In actual practise it only took me about 30 seconds to find the above
so the correct answer is (5x+6)(8x-5) ?
can be easily checked by expanding it.
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
Since the hard part was to factor it, I assumed that you would finish it.
(5x+6)(8x-5) = 0
x = -6/5 or x = 5/8
i totally dropped the ball in this one...i should have definitely know to find x!!!