Is there a greatest common factor between all of the terms in the given expression? If so, factor that out first.
(x-4)(2x+7)
Practice makes perfect
We want to completely factor the given expression. Since there is no greatest common factor, we will start by rewriting the middle term. We have a quadratic trinomial of the form ax^2+bx+c, where |a| ≠1 and there are no common factors. The coefficients of these two terms will be factors of ac whose sum must be b.
( 2x^2-x-28 ) ⇔ 2x^2+( - 1)x+( - 28)
We have that a= 2, b= - 1, and c= - 28. There are now three steps we need to follow in order to rewrite the above expression.
Find a c. Since we have that a= 2 and c= - 28, the value of a c is 2* - 28=- 56.
Find factors of a c. Since a c=- 56, which is negative, we need factors of a c to have opposite signs — one positive and one negative — in order for the product to be negative. Since b= -1, which is also negative, the absolute value of the negative factor will need to be greater than the absolute value of the positive factor, so that their sum is negative.
Factor Pair
Product
Sum
1 and -56
1* (-56) -56
1+(-56) - 55
2 and - 28
2* (-28) - 56
-2+(-28) - 26
4 and - 14
4* (- 12) - 56
4+(-14) - 10
7 and -8
7* (- 8) - 56
7+(-8) -1
Rewrite bx as two terms. Now that we know which factors are the ones to be used, we can rewrite bx as two terms.
2x^2+( - 1)x-28 ⇔ 2x^2 - 8x + 7x-28
Finally, we will factor the last expression obtained.
We can see above that after expanding and simplifying, the result is the same as the given expression. Therefore, we can be sure our solution is correct!
