(2x+y)的平方- (x+2y)的平方
=[(2x+y)- (x+2y)][(2x+y)+ (x+2y)]
=(x-y)(3x+3y)
=3(x-y)(x+y)
(a-b)^2+4ab
=a^2-2ab+b^2+4ab
=a^2+2ab+b^2
=(a+b)^2
3.14*7.8^2- 3.14*1.1^2*4
=3.14*7.8^2- 3.14*2.2^2
=3.14*(7.8^2- 2.2^2)
=3.14*(7.8+2.2)*(7.8-2.2)
=3.14*10*5.6
=31.4*5.6
=175.84