點解我計唔岩展開果d數?

1. (x+3)(2x-5)
=2x^2-5x+6x-15
=2x^2+x-15

2. (x+y)^2 - (x-y)^2
=(x+y)(x+y)-(x-y)(x-y)
=(x^2+xy+xy+y^2)-(x^2-xy-xy+y^2)
=(x^2+2xy+y^2)-(x^2-2xy+y^2)
=x^2+2xy+y^2-x^2+2xy-y^2
=x^2-x^2+2xy+2xy+y^2-y^2
=2xy+2xy
=4xy

1. (x+3)(2x-5)
=x(2x-5)+3(2x-5)
=x(2x)-x(5)+3(2x)-3(5)
=2x^2+x-15

2. (x+y)^2 - (x-y)^2
=(x+y)(x+y)-(x-y)(x-y)
=x(x+y)+y(x+y)-x(x-y)+y(x-y)
=x(x)+x(y)+y(x)+y(y)-x(x)+x(y)+y(x)-y(y)
=4xy

或咁做：
用a^2-b^2=(a+b)(a-b)
2. (x+y)^2 - (x-y)^2
=(x+y+x-y)(x+y-x+y)
=(2x)(2y)
=4xy

唔明再問la

第一條
(x+3)(2x-5)
可2睇成
(x+3)(2x) - (x+3)(5)
之後再計
2x^2+6x - (5x+15)
再後就計到
2x^2+x-15

第二條
(x+y)^2 - (x-y)^2 = [(x+y) + (x-y)] [(x+y) - (x-y)]
就會計到4xy