`(x + 10)^{10} + (x + 1/x)^{5}`
`= sum_{k = 0}^{10}.C_{10}^{k}.x^{10 - k}.10^{k} + sum_{k = 0}^{5}.C_{5}^{k}.1^{k}.(x^{5 - k})/(x^{k})`
`= sum_{k = 0}^{10}.C_{10}^{k}.x^{10 - k}.10^{k} + sum_{k = 0}^{5}.C_{5}^{k}.1^{k}.x^{5 - 2k}`
`text{Giả thuyết}`
`->` \(\left\{ \begin{array}{l}10 - k = 8\\5 - 2k = 8\end{array} \right.\)
`->` \(\left\{ \begin{array}{l}k = 2\\k = -\dfrac{3}{2} (l)\end{array} \right.\)
`text{Vậy hệ số x^8 là:} C_{10}^{2}.10^{2} + C_{5}^{2}`
