解:①+②, 得???$ 3 x-3 y=15 ，$??? 即???$ x-y=5 ④，$??? ②-③, 得???$ x+2 y=11 ⑤，$??? ⑤ - ④, 得???$ 3 y=6 ，$??? ∴???$y=2 ，$??? 把???$ y=2 $???代入④, 得???$ x=7 .$??? 再把???$ x=7，$??????$ y=2 $???代入③, 得???$ z=-2 .$??? ∴ 原方程組的解為 ???$\begin {cases}{x=7 }\\{y=2}\\{z=-2}\end {cases}$???
???$\text {解:① }-3 ×2 \text {, 得 } 5 y=5, $??? ∴???$y=1，$??? ???$\text {②}+③, \text { 得 } 8 x-5 y=7, $??? ∴???$x=\frac {3}{2}，$??? ???$\text { 再把 } x=\frac{3}{2}, y=1$??? ???$ \text { 代入③, 得 } z=-1 \text {, } $??? ∴原方程組的解為 ???$\begin {cases}{x=\frac {3}{2} }\\{y=1 }\\{z=-1}\end {cases}$???
?$\text { 解:由已知可得 }\left\{\begin{array} { l } { a + b + c = 2 , } \\{ a - b + c = - 2 , } \\{ 4 a + 2 b + c = 3 , }\end{array} 解得\left\{\begin{array}{l}a=-\frac{1}{3}, \\b=2, \\c=\frac{1}{3} .\end{array}\right.\right. $?
?$解:設(shè)這三個(gè)數(shù)\text { 分別為 } x, y, z \text {. 可得 }\left\{\begin{array} { l } { x + y = 8 3 , } \\{ y + z = 2 1 , } \\{ x + z = 3 8 , }\end{array}解得 \left\{\begin{array}{l}x=50, \\y=33, \\z=-12 .\end{array}\right.\right.$? ?$ $? ?$ $? ?$解:設(shè)一、二、三金金額分別為\ x\ 萬(wàn)元,\ y\ 萬(wàn)元和\ z\ 萬(wàn)元.\ $? ?$可得\ \left\{\begin{array}{l}10 x+20 y+30 z=41, \\ 12 x+20 y+28 z=42 \\ 14 x+25 y+40 z=54 .\end{array}\right.\ $? ?$\text {, 解這個(gè)方程 } 組, 得\ \left\{\begin{array}{l}x=1, \\ y=0.8, \\ z=0.5 .\end{array}\right.\ $?
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