Verified Answer :

meerkat18

x² + x - 20 = 0

x² = x * x
x = the sum of the factors of -20
-20 ⇒  1 x 20
            2 x 10
          - 4 x 5 ⇒ only these factors can arrive to a sum of 1

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

x - 4 = 0
x = 4

x + 5 = 0
x = -5

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