Ta có: `x= 99 => x -99=0`
`f(x) = x^21 - 100x^20 + 100x^19 -....+ 100x -1`
`f(x)= x^21- (99+1)x^20 + (99+1)x^19 -....+ (99+1)x -1`
`f(x)= x^21 - 99x^20 - x^20 + 99x^19 + x^19 -...+ 99x+ x -1`
`f(x)= x^20(x - 99) - x^19(x - 99) +...- x(x -99) +x -1`
`f(x)= x^20 . 0 - x^19 .0 +...-x . 0 +x -1`
`f(x)= x -1`
`=>f(99)= 99-1`
`=> f(99)=98`
Vậy `f(99) = 98`