49(x+y)²-9(x-y)²=7²(x+y)²-3²(x-y)² =[7(x+y)]²-[3(x-y)]² =(7x+7y)²-(3x-3y)² =[(7x+7y)+(3x-3y)][(7x+7y)-(3x-3y)] ...
49（x+y）的平方减9(x-y)的平方
49(x+y)²-9(x-y)²=7²(x+y)²-3²(x-y)² =[7(x+y)]²-[3(x-y)]² =(7x+7y)²-(3x-3y)² =[(7x+7y)+(3x-3y)][(7x+7y)-(3x-3y)] ...