Bài `4:`

`a)`

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

`=x^2+2xy+y^2+x^2-2xy+y^2`

`=(x^2+x^2)+(2xy-2xy)+(y^2+y^2)`

`=2x^2+2y^2`

`b)`

`(x-2)^2-(x+3)^2+(x-4)(x+4)`

`=(x-2-x-3)(x-2+x+3)+x^2-16`

`=-5(2x+1)+x^2-16`

`=-10x-5+x^2-16`

`=x^2-10x-21`

`c)`

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

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

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

`=(2x)^2`

`=4x^2`

`d)`

`(3x+1)^2-2(3x+1)(3x+5)+(3x+5)^2`

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

`=(-4)^2`

`=16`

`e)`

`(y-2)(y+2)(y^2+4)-(y+3)(y-3)(y^2+9)`

`=(y^2-4)(y^2+4)-(y^2-9)(y^2+9)`

`=y^4-16-(y^4-81)`

`=y^4-16-y^4+81`

`=(y^4-y^4)+(-16+81)`

`=65`

`g)`

`(x-y+z)^2+(z-y)^2+2(x-y+z)(y-z)`

`=(x-y+z)^2+2(x-y+z)(y-z)+(y-z)^2`

`=(x-y+z+y-z)^2`

`=x^2`

`#owen`