Question 582095: I need help factoring. I just don't get it.
8x^2y+34xy-84y
Found 2 solutions by jim_thompson5910, richard1234:
Answer by jim_thompson5910(35256)   (Show Source):
You can put this solution on YOUR website!
 Start with the given expression.
 Factor out the GCF  .
Now let's try to factor the inner expression 
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Looking at the expression , we can see that the first coefficient is  , the second coefficient is  , and the last term is  .
Now multiply the first coefficient by the last term to get  .
Now the question is: what two whole numbers multiply to  (the previous product) and add to the second coefficient ?
To find these two numbers, we need to list all of the factors of (the previous product).
Factors of :
1,2,3,4,6,7,8,12,14,21,24,28,42,56,84,168
-1,-2,-3,-4,-6,-7,-8,-12,-14,-21,-24,-28,-42,-56,-84,-168
Note: list the negative of each factor. This will allow us to find all possible combinations.
These factors pair up and multiply to .
1*(-168) = -168
2*(-84) = -168
3*(-56) = -168
4*(-42) = -168
6*(-28) = -168
7*(-24) = -168
8*(-21) = -168
12*(-14) = -168
(-1)*(168) = -168
(-2)*(84) = -168
(-3)*(56) = -168
(-4)*(42) = -168
(-6)*(28) = -168
(-7)*(24) = -168
(-8)*(21) = -168
(-12)*(14) = -168
Now let's add up each pair of factors to see if one pair adds to the middle coefficient :
| First Number | Second Number | Sum | | 1 | -168 | 1+(-168)=-167 | | 2 | -84 | 2+(-84)=-82 | | 3 | -56 | 3+(-56)=-53 | | 4 | -42 | 4+(-42)=-38 | | 6 | -28 | 6+(-28)=-22 | | 7 | -24 | 7+(-24)=-17 | | 8 | -21 | 8+(-21)=-13 | | 12 | -14 | 12+(-14)=-2 | | -1 | 168 | -1+168=167 | | -2 | 84 | -2+84=82 | | -3 | 56 | -3+56=53 | | -4 | 42 | -4+42=38 | | -6 | 28 | -6+28=22 | | -7 | 24 | -7+24=17 | | -8 | 21 | -8+21=13 | | -12 | 14 | -12+14=2 |
From the table, we can see that the two numbers  and  add to (the middle coefficient).
So the two numbers and both multiply to and add to
Now replace the middle term  with  . Remember, and add to . So this shows us that  .
 Replace the second term with .
 Group the terms into two pairs.
 Factor out the GCF  from the first group.
 Factor out  from the second group. The goal of this step is to make the terms in the second parenthesis equal to the terms in the first parenthesis.
 Combine like terms. Or factor out the common term 
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So then factors further to 
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Answer:
So completely factors to .
In other words,  .
Note: you can check the answer by expanding to get or by graphing the original expression and the answer (the two graphs should be identical).
Answer by richard1234(7193)  (Show Source):
You can put this solution on YOUR website! First you can factor out 2y:
)
The 4x^2 + 17x - 42 is a bit tricky to factor; the best way to learn is by experience and simply being able to "see" the correct factorization.
4x^2 + 17x - 42 can be factored into a product (ax + b)(cx + d). We want ac = 4, so a,c can equal 2 or 1,4. However, if a = c = +/- 2, the x coefficient will be even, so try a=1, c=4. We can kind of "guess and check" based on the factorization of -42 to obtain the factorization
(4x - 7)) , in which the complete answer would be (4x-7)) .
Again, this takes lots of practice. Another way you could factor it is to find the roots of 4x^2 + 17x - 42 by the quadratic formula:
Then you could factor it as (x + 6)) . However, the leading coefficient is 4 so we can multiply the first term by 4.
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