:4x(x-4)+21=5
4(x²-4x+4)=0
(x-2)²=0
x=2
9x^2=(2x-5)^2
(3x)²-(2x-5)²=0
(3x+2x-5)(3x-2x+5)=0
5(x-1)(x+5)=0
x=1,x=-5计算：（4x-x^3)/(x+2)（-x^2y^3+4xy^2-4y)/(xy-2)[(2x-1)^2-(x+1)^2]/(x-2)(x^3+x^2-x-1)/(x-1)