x（x-1）=90
x^2-x=90
x^2-x+1/4=90+1/4
(x-1/2)^2=361/4
x-1/2=±√361/2
x=(1±√361)/2
x1=10 x2=-9
配为x^2-x+1/4=361/4
即(x-1/2)^2=361/4
解得 x=10或x=-9
x（x-1）=90
x平方-x-90=0
(x-10)(x+9)=0
x1=10
x2=-9
x（x-1）=90
x^2 -x-90=0
(x-10)(x+9)=0
x1=10
x2=-9
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