Đáp án:
`a,` `(y+3x)(9x^2-3xy+y^2)`
`=y(9x^2-3xy+y^2)+3x(9x^2-3xy+y^2)`
`=9x^2y-3xy^2+y^3+27x^3-9x^2y+3xy^2`
`=-3xy^2+y^3+27x^3+3xy^2`
`=y^3+27x^3`
`b,` `(4-x/2)(x^2/4+2x+16)`
`=4(x^2/4+2x+16)-x/2(x^2/4+2x+16)`
`=x^2+8x+64-x/2 . (x^2+8x+64)/4`
`=x^2+8x+64- (x(x^2+8x+64))/2.4`
`=x^2+8x+64-(x^3+8x^2+64x)/8`
`=(8x^2+64x-(x^3+8x^2+64x))/8+64`
`=(8x^2+64x-x^3-8x^2-64x)/8+64`
`=(64x-x^3-64x)/8+64`
`=(-x^3)/8+64`
`=64-(x^3)/8`