How do you factor the expression x^2+4x-21?

2 Answers
Mar 6, 2018

(x+7)(x-3)

Explanation:

To factorise any quadratic polynomial , you could use 2 methods.
Here's the easy one
Let the quadratic polynomial be ax^2+bx+c

▪multiply a" and "c , then factorise the result into some factors, say y and z, y+z=|b|
x^2+4x-21 (a×c=-21=-3×7)

x^2+7x-3x-21

x (x+7)-3 (x+7)

(x+7)(x-3)

The other method is a direct application of quadratic formula.

Mar 6, 2018

the answer is (x+7)(x-3)

Explanation:

there's an easier way to find this but only when it's x^2. since you know that two numbers have to multiply to give you -21 and add to give you 4 when you find it, you don't have to go through the whole process and you can just put them in the parenthesis since there's no number in front of x