x^2 - x = 20
x^2-x-20 =0
x^2-16-x-4=0
(x^2-4^2)-(x+4)=0
(x+4)(x-4)-(x+4)=0
(x+4)(x-4-1)=0
(x+4)(x-5)=0
x = -4或 x=5
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