a) `x²+4y²-25+4xy`
`= (x²+4xy+4y²)-25`
`= [x²+2.x.2y+(2y)²]-5²`
`= (x+2y)²-5²`
`= (x+2y-5)(x+2y+5)`
b) `x³-2x²+4xy-2y²-y³`
`= (x³-y³)-(2x²-4xy+2y²)`
`= (x³-y³)-(2x²-2xy-2xy+2y²)`
`= (x³-y³)-[(2x²-2xy)-(2xy-2y²)]`
`= (x³-y³)-[2x(x-y)-2y(x-y)]`
`= (x-y)(x²+xy+y²)-(x-y)(2x-2y)`
`= (x-y)(x²+xy+y²-2x-2y)`