（x+2)的2次幂=x²+4x+4
(7+m)的2次幂=m²+14m+49
(5x-2y)的2次幂=(5x)²-2*5x*2y+(2y)²=25x²-20xy+4y²
(2x-2分之y)的2次幂=(2x)²-2*2x*y/2+(y/2)²=4x²-2xy+y²/4
(a-b-c)²
=[(a-b)-c]²
=(a-b)²-2(a-b)c+c²
=a²-2ab+b²-2ac+2bc+c²