.（x-5）^2+2(x-5)+1=0
（x-5+1）^2=0
(x-4)^2=0
x=4
.9(x-1)^2-4(x+2)^2=0
[3(x-1)+2(x+2)][3(x-1)-2(x+2)]=0
(5x+1)(x-7)=0
x=-1/5 或 x=7