Cho hệ phương trình

- Hướng dẫn giải

Ta có 

$\left\{\begin{array}{l}2x-7y=8\\ 10x+3y=21\end{array}\right.\iff \left\{\begin{array}{l}x=\frac{8+7y}{2}\\ 10.\left(\frac{8+7y}{2}\right)+3y=21\end{array}\right.$

$\iff \left\{\begin{array}{l}x=\frac{8+7y}{2}\\ 40+35y+3y=21\end{array}\right.\iff \left\{\begin{array}{l}x=\frac{8+7y}{2}\\ 38y=-19\end{array}\right.$

$\iff \left\{\begin{array}{l}x=\frac{9}{4}\\ y=-\frac{1}{2}\end{array}\right.\Rightarrow x+y=\frac{9}{4}-\frac{1}{2}=\frac{7}{4}$

Đáp án: D