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第45頁

信息發(fā)布者：
①
①
②
②
解:②-①得??????$： \frac {2}{3} x=3 ， $??????
即??????$ x=\frac {9}{2} ，$??????
將??????$ x=\frac {9}{2} $??????代入②
得??????$： y=-9 ，$??????
則方程組的解為??????$\begin {cases}{x=\frac {9}{2} }\\{y=-9}\end {cases}$??????

解:由①得??????$： x=-y+8③ ，$??????
把③代入②得??????$： 3(-y+8)-2 y=-1 ，$??????
解得??????$： y=5 ，$??????
把??????$ y=5 $??????代入 ③得??????$： x=-5+8=3 ，$??????
原方程組的解為??????$： \begin {cases}{x=3 }\\{y=5}\end {cases}$??????
解:由題意得??????$: \begin{cases}{2 a-b=1 \text { ① } }\\{a+2 b=8②}\end{cases}$??????
??????$\text { ① } ×2+\text { ②得: } 5 a=10 $??????
∴??????$a=2 $??????
??????$\text { 把 } a=2 \text { 代入①得: } 2 ×2-b=1 $??????
∴??????$b=3$??????

解??????$：\begin {cases}{x+y=a①}\\{5 x-3 y=-a②}\end {cases}$??????
??????$\text {①} ×3+\text { ②得: } 8 x=2 a $??????
∴??????$x=\frac {a}{4} $??????
∴??????$\frac {a}{4}+y=a $??????
∴??????$y=\frac {3\ \mathrm {a}}{4} $??????
∴??????$\begin {cases}{x=\frac {a}{4} }\\{y=\frac {3\ \mathrm {a}}{4}}\end {cases}$??????

解??????$：\begin {cases}{2 x+y=7①}\\{x+2 y=8②}\end {cases}$??????
??????$① ×2 ， $??????得??????$4 x+2 y=14③$??????
③ -②, 得??????$3 x=6 ，$??????∴??????$x=2$??????
將??????$ x=2 $??????代入 ①, 得??????$2 ×2+y=7 $??????
∴??????$y=3$??????
所以方程組的解是??????$\begin {cases}{x=2 }\\{y=3}\end {cases}$??????
∴??????$\frac {x-y}{x+y}=\frac {2-3}{2+3}=-\frac {1}{5}$??????

????$解:\because a//b\ $????
????$\therefore \angle 1+\angle 2=180^{\circ}\ $????
????$\therefore \angle 1=180^{\circ}-\angle 2\ $????
????$\because 2 \angle 2-\angle 1=30^{\circ}\ $????
????$\therefore 2 \angle 2-\left(180^{\circ}-\angle 2\right)=30^{\circ}\ $????
????$2 \angle 2-180^{\circ}+\angle 2=30^{\circ}\ $????
????$3 \angle 2=210^{\circ}\ $????
????$\therefore \angle 2=70^{\circ}\ $????
????$\therefore \angle 3=180^{\circ}-\angle 2=180^{\circ}-70^{\circ} =110^{\circ}$????
????$ $????
????$\begin {cases}{x=2}\\{y=-1}\end {cases}$????
??$解:\left\{\begin{array}{r}2 x+3 y=15 m① \\5 x-3 y=-m②\end{array}\right.\ $??
??$\text { ① }+ \text { ② 得 } 7 x=14 m\ $??
??$x=2 m\ $??
??$將\ x=2 m\ 代入①得\ 4 m+3 y=15 m\ $??
??$y=\frac{11}{3} m$??
??$故原方程組的解為\ \left\{\begin{array}{l}x=2 m \\ y=\frac{11}{3} m\end{array}\right. .$??
??$ $??
????$解:解方程組\ \left\{\begin{array}{l}2 x+5 y=-6 \\ 3 x-5 y=16\end{array}\right. ,得\left\{\begin{array}{l}x=2 \\y=-2\end{array}\right.$????
????$把\ \left\{\begin{array}{l}x=2 \\ y=-2\end{array}\right.\ 代入\ \left\{\begin{array}{l}a x-b y=-4 \\ b x+a y=-8\end{array}\right.\ 可得\left\{\begin{array}{l}2 a+2 b=-4 \\2 b-2 a=-8\end{array}\right.$????
????$解之可得\left\{\begin{array}{l}a=1 \\b=-3\end{array}\right.$????
????$把\ \left\{\begin{array}{l}a=1 \\ b=-3\end{array}\right.\ 代入\ 3 {a}+2  , 得3 a+2 b=-3$????
????$ $????
????$ $????
????$解:\because(3 x-2 y-5)^{2}+\sqrt{x-2 y+3}=0\ $????
????$\therefore \left\{\begin{array}{l}3 x-2 y-5=0\\ x-2 y+3=0\end{array}\right.$????
????$解得\left\{\begin{array}{l}x=4 \\y=\frac{7}{2}\end{array}\right.\ $????
????$\therefore x y-5=4 \times \frac{7}{2}-5=9,\ $????
????$\therefore x y-5 \text { 的平方根是 } \pm 3 .$????
????$ $????
????$ $????
??$解:已知\ \left\{\begin{array}{l}2(x+y)+3(x-y)=3① \\ 7(x+y)-3(x-y)=24②\end{array}\right.\ \ $??
??$①+ ②得:\ 9(x+y)=27 ,即\ x+y=3③\ $??
??$將③代入①中:\ 2 \times 3+3(x-y)=3 ,即\ x-y=-1 .$??
??$由\ \left\{\begin{array}{l}x+y=3 \\ x-y=-1\end{array}\right. , 解得\ \left\{\begin{array}{l}x=1 \\ y=2\end{array}\right.\ $??

??$ $??
??$解: 由原方程組, 得\ \left\{\begin{array}{l}2 x+y=3 \text {, ① } \\ 2 x-y=5 \text {. ② }\end{array}\right.\ $??
??$由 ① +②, 得\ 4 x=8 , 解得\ x=2 .$??
??$由①-②, 得\ 2 y=-2 , 解得\ y=-1 .$??
??$所以原方程組的解是\ \left\{\begin{array}{l}x=2, \\ y=-1 .\end{array}\right.\ $??

??$ $??
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