Đáp án:
1/ `(x-2y)^2+(x+2y)^2`
`= (x^2-4xy+4y^2)+(x^2+4xy+4y^2)`
`= x^2-4xy+4y^2+x^2+4xy+4y^2`
`= (x^2+x^2)+(-4xy+4xy)+(4y^2+4y^2)`
`= 2x^2+8y^2`
2/ `(a-b)^2+(a+b)^2`
`= (a^2-2ab+b^2)+(a^2+2ab+b^2)`
`= a^2-2ab+b^2+a^2+2ab+b^2`
`= (a^2+a^2)+(-2ab+2ab)+(b^2+b^2)`
`= 2a^2+2b^2`
3/ `(a+b)^2-(a-b)^2`
`= (a^2+2ab+b^2)-(a^2-2ab+b^2)`
`= a^2+2ab+b^2-a^2+2ab-b^2`
`= (a^2-a^2)+(2ab+2ab)+(b^2-b^2)`
`= 4ab`
4/ `(x+5y)^2+(5y-x)2`
`= (x^2+10xy+25^2)-(25y^2-10xy+x^2)`
`= x^2+10xy+25y^2-25y^2-10xy+x^2`
`= (x^2+x^2)+(10xy-10xy)+(25y^2-25y^2)`
`= 2x^2`
5/ `(x-2y)(x+2y)-(x+y)^2`
`= (x^2-4y^2)-(x^2+2xy+y^2)`
`= x^2-4y^2-x^2-2xy-y^2`
`= (x^2-x^2)-(4y^2-y^2)-2xy`
`= -5y^2-2xy`
