Tính giá trị: B = x^15 - 5x^14 + 8x^13 - 8^12 + ... - 8^2 + 8x - 5 với x = 7

* Hướng dẫn giải

$\begin{array}{l}\mathrm{B}={\mathrm{x}}^{15}-8{\mathrm{x}}^{14}+8{\mathrm{x}}^{13}-8{\mathrm{x}}^{12}+\mathrm{.....}-8{\mathrm{x}}^2+8\mathrm{x}-5\\ ={\mathrm{x}}^{15}-7{\mathrm{x}}^{14}-{\mathrm{x}}^{14}+\mathrm{......}-7{\mathrm{x}}^2-{\mathrm{x}}^2+7\mathrm{x}+\mathrm{x}-7+2\\ =\left(\mathrm{x}-7\right)\left({\mathrm{x}}^{14}-{\mathrm{x}}^{13}+\mathrm{.....}-\mathrm{x}+1\right)+2\end{array}$