x的平方减4x加1等于0x的平方减4x = -1 代数式(2x-3)^2-(x+y)(x-y)-y^2=4x^2-12x+9-(x^2-y^2)-y^2=3(x^2-4x+3)=3 x ( -1+3) = 3 x 2 = 6
