X*X-X=870

x^2-x-870=0

x^2-x+1/4=870+1/4

(x-1/2)^2=3481/4

x-1/2=59/2，x=30
或
x-1/2=-59/2,x=-29

x=30或-29

X^2-X=870
X^2-X+1/4=870+1/4
X^2-2*X*1/2+(1/2)^2=3481/4
(X-1/2)^2=(±59/2)^2
X-1/2=±59/2
X=1/2±59/2
所以X1=30,X2=-29

X*X-X=870
x^2-x-870=0
(x-30)(x+29)=0
x1=30
x2=-29

用求根公式！