How do you solve the equation x^2+2x-120=0 by completing the square?

1 Answer
George C.
Dec 4, 2016

x=10 or x = -12

Explanation:

The difference of squares identity can be written:

a^2-b^2 = (a-b)(a+b)

So we find:

0 = x^2+2x-120

color(white)(0) = x^2+2x+1-121

color(white)(0) = (x+1)^2-11^2

color(white)(0) = ((x+1)-11)((x+1)+11)

color(white)(0) = (x-10)(x+12)

Hence:

x = 10 or x = -12

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