=4(x-y)²-4(x-y)+1
=[2(x-y)-1]²
=(2x-2y-1)²4(x-y)²-4(x-y)+1到[2(x-y)-1]²是什么？=[2(x-y)]²-2×2(x-y)×1+1²=[2(x-y)-1]²a²-2 a b+b²=(a -b)²