$解:\left\{\begin{array}{l}x+y=3① \\5 x-3(x+y)=1②\end{array},\right.\\① 代入②得 5 x-3 \times 3=1 ,解得 x=2 ,\\将 x=2 代入①得, 2+y=3 ,解得 y=1 ,\\ \therefore 方程组的解为 \left\{\begin{array}{l}x=2 \\ y=1\end{array}\right. \\$ $解:\left\{\begin{array}{l}3 x+2 y=1① \\4 x-y=-6②\end{array},\right.\\② \times 2 得, 8 x-2 y=-12 ③, ①+③ 得 11 x=-11 ,\\解得 x=-1 ,将 x=-1 代入②得, y=2 , \\\therefore 方程组的解为 \left\{\begin{array}{l}x=-1 \\ y=2\end{array}\right. \\$ $解:\because|x-2 y+7|+(x+y-5)^{2}=0, \\\therefore\left\{\begin{array}{l}x-2 y=-7 \\x+y=5\end{array},\right.,解得: \left\{\begin{array}{l}x=1 \\ y=4\end{array}\right. $ $解:因为 x 、 y 的值互为相反数, 所以 x=-y . \\将 x=-y 代入方程组, 得 \left\{\begin{array}{l}-3 y-5 y=2 a①, \\ -2 y+7 y=-10②,\end{array}\right. \\解 ② 得 5 y=-10 ,所以 y=-2 . 所以 x=2 . \\把 y=-2 代人 ①, 得 -8 \times (-2)=2 a , 解得 a=8 . 所以 a 的值为 8 ,\\方程组的解为 \left\{\begin{array}{l}x=2, \\ y=-2\end{array}\right. \\$
$解:方程组 \left\{\begin{array}{l}2 x+y=7①, \\ x=y-1②,\end{array}\right. \\ 把 ②代入 ①, 得 2(y-1)+ y=7 , 解得 y=3 . 把 y=3 代入 ①, 解得 x=2 . \\把 \left\{\begin{array}{l}x=2, \\ y=3\end{array}\right. 代入方程 a x+y=4 , 得 2 a+3=4 , 解得 a= \frac{1}{2} \\$ $解: 将 \left\{\begin{array}{l}x=3, \\ y=-2,\end{array},\left\{\begin{array}{l}x=-2, \\ y=2\end{array}\right.\right. 代人 a x+b y=2 中,\\得 \left\{\begin{array}{l}3 a-2 b=2, \\ -2 a+2 b=2,\end{array}\right. 解得 \left\{\begin{array}{l}a=4, \\ b=5 .\end{array}\right. \\将 \left\{\begin{array}{l}x=3, \\ y=-2\end{array}\right. 代入 c x- 7 y=8 中, 解得 c=-2 $
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