$解:将②代入①,得\frac{4t+10}{3}=3t,$ $解得t=2. $ $将t=2代入②,得s=3. $ $所以方程组的解为\begin{cases}{s=3,}\\{t=2.}\end{cases}$
$解:由②,得x=-4y-15,③ $ $将③代入①,$ $得3(-4y-15)-5y=6,$ $解得y=-3. $ $将y=-3代入③,得x=-3. $ $所以方程组的解为\begin{cases}{x=-3,}\\{y=-3.}\end{cases}$ $解:由①,得y=-2x+4,③ $ $将③代入②,得3x-(-2x+4)-6=0,$ $解得x=2. $ $将x=2代入③,得y=0. $ $所以方程组的解为\begin{cases}{x=2,}\\{y=0.}\end{cases}$
$解:\begin{cases}{2x+4y=5,①}\\{x=1-y,②}\end{cases}$ $把②带入①,得2(1-y)+4y=5,$ $解得y=\frac {3}{2}.$ $把y=\frac {3}{2}代入②,得x=1-\frac {3}{2}=-\frac {1}{2}.$ $所以方程组的解为\begin{cases}{x=-\dfrac {1}{2},}\\{y=\dfrac {3}{2}.}\end{cases}$
$解:\begin{cases}{2x-y=3,①}\\{3x+2y=8,②}\end{cases}$ $ 由①,得y=2x-3,③ $ $把③代入②,得3x+2(2x-3)=8,$ $解得x=2.$ $把x=2代入③,得y=2×2-3=1. $ $所以方程组的解为\begin{cases}{x=2,}\\{y=1.}\end{cases}$
$解:\begin{cases}{3u+2v=-2,①}\\{3u-v=4,②}\end{cases}$ $由②,得v=3u-4,③ $ $将③代入①,得3u+2(3u-4)=-2, $ $解得u=\frac {2}{3}.$ $将u=\frac {2}{3}代入③,得v=-2.$ $所以方程组的解为\begin{cases}{u=\frac {2}{3},}\\{v=-2.}\end{cases}$
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