giúp em với cảm ơn mn

Bài `4:`

`1,(x-y)(x+y)=x^2-y^2`

`2, (x-3y)(x+3y)=x^2-9y^2`

`3, (2x-5y)(2x+5y)=4x^2-25y^2`

`4, (x^2+y)(x^2-y)`

`=x^4-y^2`

`5, (2x-3y^2)(2x+3y^2)`

`=4x^2-9y^4`

`6, (-x-1/2y)(-x+1/2y)`

`=x^2-1/4y^2`

`7, (1/2x-2/5y^3)(2/5y^3+1/2x)`

`=(1/2x-2/5y^3)(1/2x+2/5y^3)`

`=1/4x^2-4/25y^6`

`8, (x-6)(6+x)`

`=x^2-36`

`9, (2x+1)(2x-1)`

`=4x^2-1`

`10, (x-2y)(2y+x)`

`=x^2-4y^2`

`11, (3x-y^2)(3x+y^2)`

`=9x^2-y^4`

`12, (x^2-2y)(x^2+2y)`

`=x^4-4y^2`

`13, (5x-3y)(3y+5x)`

`=25x^2-9y^2`

`14, (1/x-5)(1/x+5)`

`=1/(x^2)-25`

`15, (x-3/2)(x+3/2)`

`=x^2-9/4`

Áp dụng `HĐT:a^2-b^2=(a-b)(a+b)`