Giải các hệ phương trình sau bằng phương pháp cộng đại số:

a, $\left\{\begin{array}{l}3x+y=3\\ 2x-y=7\end{array}\right.$;     b, $\left\{\begin{array}{l}2x+5y=8\\ 2x-3y=0\end{array}\right.$;      c, $\left\{\begin{array}{l}4x+3y=6\\ 2x+y=4\end{array}\right.$;      d, $\left\{\begin{array}{l}2x+3y=-2\\ 3x-2y=-3\end{array}\right.$  ;     e, $\left\{\begin{array}{l}0,3x+0,5y=3\\ 1,5x-2y=1,5\end{array}\right.$

Giải: 

a, $\left\{\begin{array}{l}3x+y=3\\ 2x-y=7\end{array}\right.$$\iff$$\left\{\begin{array}{l}5x=10\\ 2x-y=7\end{array}\right.$$\iff$$\left\{\begin{array}{l}x=2\\ y=2x-7\end{array}\right.$$\iff$$\left\{\begin{array}{l}x=2\\ y=-3\end{array}\right.$

b, $\left\{\begin{array}{l}2x+5=8\\ 2x-3y=0\end{array}\right.$$\iff$$\left\{\begin{array}{l}2x+5y=8\\ 8y=8\end{array}\right.$$\iff$$\left\{\begin{array}{l}2x+5y=8\\ y=1\end{array}\right.$$\iff$$\left\{\begin{array}{l}x=\begin{array}{c}3\\ 2\end{array}\\ y=1\end{array}\right.$

c, $\left\{\begin{array}{l}4x+3y=6\\ 2x+y=4\end{array}\right.$$\iff$$\left\{\begin{array}{l}4x+3y=6\\ 4x+2y=8\end{array}\right.$$\iff$$\left\{\begin{array}{l}4x+3y=6\\ y=-2\end{array}\right.$$\iff$$\left\{\begin{array}{l}x=3\\ y=-2\end{array}\right.$

d, $\left\{\begin{array}{l}2x+3y=-2\\ 3x-2y=-3\end{array}\right.$$\iff$$\left\{\begin{array}{l}4x+6y=-4\\ 9x-6y=-9\end{array}\right.\iff \left\{\begin{array}{l}4x+6y=-4\\ 13x=-13\end{array}\right.\iff \left\{\begin{array}{l}x=-1\\ y=0\end{array}\right.$

e, $\left\{\begin{array}{l}0,3x+0,5y=3\\ 1,5x-2y=1,5\end{array}\right.\iff \left\{\begin{array}{l}1,5x+2,5y=15\\ 1,5x-2y=1,5\end{array}\right.\iff \left\{\begin{array}{l}1,5x+2,5y=15\\ 4,5y=13,5\end{array}\iff \left\{\begin{array}{l}1,5x=15-2,5.3\\ y=3\end{array}\iff \left\{\begin{array}{l}1,5x=7,5\\ y=3\end{array}\iff \left\{\begin{array}{l}x=5\\ y=3\end{array}\right.\right.\right.\right.$