A (a-b)^2=a^2-2ab+b^2B (2x-y)^2=4x^2-4xy+y^2C (a+2b)^2=a^2+4ab+4b^2D (x+3)^2=x^2+6x+9D是对的
D(x+3)^2=x^2+6x+9