Questions on Algebra: Polynomials, rational expressions and equations answered by real tutors!

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Question 152454: factor by grouping:
8x^2+18x-35
: factor by grouping:
8x^2+18x-35

Answer by Edwin McCravy(2086) About Me  (Show Source):
You can put this solution on YOUR website!
factor by grouping:
8x^2+18x-35

Multiply the 8 by the -35 and get -280
Write down all the pairs of factors of its absolute value,
280:

280*1
140*2
70*4
56*5
40*7
35*8
28*10
20*14

Now since the LAST SIGN in 8x^2+18x-35
is MINUS, we SUBTRACT the two 
factors in absolute value:

[Note: If the last term had been PLUS, we 
would have ADDED the two factors in 
absolute value.]

280-1=279
140-2=138
70-4-66
56-5=51
40-7=33
35-8=27
28-10=18
20-14=6

Now we look at the middle term of 8x^2+18x-35,
which is 18 and we look at the 
list of differences and find the
absolute value of 18 in the next to
last difference, 28-10=18.

So we substitute 28-10 for the 18:

8x^2+18x-35
8x^2+(28-10)x-35

Now we remove the parentheses by distributing:

8x^2+28x-10x-35

Factor 4x out of the first two terms
Factor -5 out of the last two terms:

4x*(2x+7)-5(2x+7)

Now factor out (2x+7)

(2x+7)(4x-5)

Edwin