4+12(x-y)+9(x-y)的2次方=(2+3x-3y)^2

不懂可追问,有帮助请采纳,谢谢!请说详细点完全平方式我看见有人这么写的：4+12(x-y)+9(x-y)^2=2^2+2*2*[3(x-y)]+[3(x-y)]^2=[2+3(x-y)]^2=(3x-3y+2)^2但为什么2^2+2*2*[3(x-y)]+[3(x-y)]^2=[2+3(x-y)]^2不是一样的吗