`a) (5x-2y) (x^2-xy+1)`
`= 5x.(x^2-xy+1) - 2y.(x^2-xy+1)`
`=5x^3-5x^2y+5x-2x^2y+2xy^2-2y`
`=5x^3-7x^2y+5x+2xy^2-2y`
`b) (x-1) (x+1) (x+2)`
`=(x^2-1). (x+2)`
`=x^2. (x+2) -1. (x+2)`
`=x^3+2x^2-x-2`
`c) (x- 1/2) (x+ 1/2) (4x-1)`
`= (x^2-(1/2)^2) . (4x-1)`
`=(x^2-1/4). (4x-1)`
`=x^2. (4x-1) - 1/4 . (4x-1)`
`=4x^3-x^2-x+1/4`