How do you factor x²-9x plus 20?

Answer 1

You can do this by finding two numbers whose sum is -9, and whose multiple is 20. Because their product is positive and their sum is negative, you know they'll both be negative numbers. So first find all of the factors of 20:

2 * 10 = 20

5 * 4 = 20

Now find which of those adds up to 9:

2 + 10 = 12

5 + 4 = 9

So we know five and four are the numbers we want. Remember though that they'll both be negative. So our end result is:

x2 - 9x + 20 = (x - 4)(x - 5)