25x²-10x(x-y)+(x-y)² =[5x-(x-y)]²=(5x-x+y)²=(4x+y)²为什么252-10x(x-y)+(x-y)2 =[5x-(x-y)]2 ？用的是完全平方差公式：a²-2ab+b²=(a-b)²