`a) VT=(x+y)^2`
`=x^2+2xy+y^2`
`=(x^2-2xy+y^2)+4xy`
`=(x-y)^2+4xy=VP`
`⇒đpcm`
`b)VT=(x-y)^2`
`=x^2-2xy+y^2`
`=(x^2+2xy+y^2)-4xy`
`=(x+y)^2-4xy=VP`
`⇒đpcm`
`c) (x+y)^2+(x-y)^2`
`=(x^2+2xy+y^2)+(x^2-2xy+y^2)`
`=x^2+2xy+y^2+x^2-2xy+y^2`
`=2x^2+2y^2`
`=2(x^2+y^2)=VT`
`⇒đpcm`
`d) VT=x^2+y^2`
`=(x^2+2xy+y^2)-2xy`
`=(x+y)^2-2xy=VP`
`⇒đpcm`
`e)VT= (x+y)^2-(x+y)(x-y)`
`=x^2+2xy+y^2-x^2+y^2`
`=2xy+2y^2`
`=2y(x+y)=VP`
`⇒đpcm`
`g) VT=(x-y)^2`
`=x^2-2xy+y^2`
`=y^2-2xy+x^2`
`=(y-x)^2=VP`
`⇒đpcm`
`h)VP=(x-y)^2+2xy`
`=(x^2-2xy+y^2)+2xy`
`=x^2+y^2=VT`
`⇒đpcm`
