x²-6ax+9a²-4b²=0 变为:(x-3a)^2-(2b)^2=0(x-3a+2b)(x-3a-2b)=0 所以x=3a-2b或x=3a+2b
x^2-6ax+(3a)^2-(2b)^2=0(x-3a)^2=(2b)^2x-3a=2b或-2bx1=3a+2b x2=3a-2b