Bài `3:`

Vì `x=9` nên ta có `10=x+1` 

Ta có:

`C=x^14-10x^13+10x^12-10x^11+...+10x^2-10x+10`

`C=x^14-(x+1)x^13+(x+1)x^12-(x+1)x^11+...+(x+1)x^2-(x+1)x+10`

`C=x^14-(x^14+x^13)+(x^13+x^12)-(x^12+x^11)+...+(x^3+x^2)-(x^2+x)+10`

`C=x^14-x^14-x^13+x^13+x^12-x^12-x^11+...+x^3+x^2-x^2-x+10`

`C=-x+10`

Thay `x=9` vào `C`, ta có:

`C=-9+10=1`

Vậy `C=1` tại `x=9`

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