Bài `5`:
a) `51^3`
`=(50+1)^3`
`=50^3+3.50^{2}.1+3.50.1^2+1^3`
`=125000+7500+150+1`
`=132500+151`
`=132651`
b) `89^3+33.89^2+3.121.89+11^3`
`=89^3+3.89^{2}.11+3.89.11^2+11^3`
`=(89+11)^3`
`=100^3`
`=1000000`
c) `23^3-9.23^2+27.23-27`
`=23^3-3.23^{2}.3+3.23.3^2-3^3`
`=(23-3)^3`
`=20^3`
`=8000`
Bài `6`:
a) `(x+2)^3`
`=x^3+3.x^{2}.2+3.x.2^2+2^3`
`=x^3+6x^2+12x+8`
b) `(x-1/2)^3`
`=x^3-3.x^{2}.(1)/(2)+3.x.(1/2)^2-(1/2)^3`
`=x^3-3/(2)x^2+3/(4)x-1/8`
c) `(x-2y)^3`
`=x^3-3.x^{2}.2y+3.x.(2y)^2-(2y)^3`
`=x^3-6x^2y+12xy^2-8y^3`
d) `(x+(y^2)/(2))^3`
`=x^3+3.x^{2}.(y^2)/(2)+3.x.(y^2/2)^2+(y^2/2)^3`
`=x^3+3/(2)x^2y^2+3/(4)xy^4+y^6/8`
