Is there a greatest common factor between all of the terms in the given expression? If so, you should factor that out first.
9
Practice makes perfect
We want to completely factor the left-hand-side expression to find the missing value in the following statement.
7x^2-61x-18=(7x+2)(x- )
Here we have a quadratic trinomial of the form ax^2+bx+c, where |a| ≠ 1 and there are no common factors. To factor this expression, we will rewrite the middle term, bx, as two terms. The coefficients of these two terms will be factors of ac whose sum must be b.
7x^2-61x-18 ⇔ 7x^2+(- 61)x+(- 18)
We have that a= 7, b=- 61, and c=- 18. There are now three steps we need to follow in order to rewrite the above expression.
Find a c. Since we have that a= 7 and c=- 18, the value of a c is 7* - 18=- 126.
Find factors of a c. Since ac=- 126, 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=- 61, 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.
We can see above that after expanding and simplifying, the result is the same as the expression we had in the beginning. Therefore, we can be sure our solution is correct!
