SOLUTION: How do I factor 4x^2-31x+21? Thanks!!

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Question 727713: How do I factor 4x^2-31x+21? Thanks!!
Answer by KMST(5328) About Me  (Show Source):
You can put this solution on YOUR website!
It is not easy, but 4x%5E2-31x%2B21 can be factored as a product of two binomials with integer coefficients.
Some (not all) quadratic polynomials can be factored.
Of the ones than can be factored, some (not all) have factors with integer coefficients.
Here's how to factor 4x%5E2-31x%2B21.

STEP 1
Multiply the leading coefficient times the independent (constant) term.
4%2A21=84

STEP 2
Look for pairs of integers that multiply to yield that product, but do not worry about negative signs yet.
There are 6 such pairs of factors.
(You would list them the same way if the product had been -84)
1%2A84=84
2%2A42=84
3%2A28=84
4%2A21=84
6%2A14=84
7%2A12=84

STEP 3
Find the pair of factors (including negative signs as needed) whose product is the product found above, and whose sum equals the coefficient of the term in x.
We are looking for factors whose product is 84 and whose sum is -31.
They will both be negative, so their absolute values will add to 31.
1%2B84=85
2%2B42=44
highlight%283%2B28=31%29
4%2B21=25
6%2B14=20
7%2B12=19
The pair of factors we are looking for is -3 and -28

STEP 4
Write the factors as coefficients of terms in x, and re-write your polynomial with those two new terms instead of the original term in x.
We write-3x and -28x and using those two expressions to replace -31x in the polynomial to get
4x%5E2-31x%2B21=4x%5E2-3x-28x%2B21

STEP 4
Factor by parts, taking out common factors for pairs of the terms.
4x%5E2-28x=4x%28x-7%29 and -3x%2B21=-3%28x-7%29 so to start we write
4x%5E2-3x-28x%2B21=%284x%5E2-28x%29%2B%28-3x%2B21%29=4x%28x-7%29-3%28x-7%29
Then we take out %28x-7%29 as common factor to finish and we have the factoring as