第52页 - 人教版数学补充习题七年级上下册答案

$\frac {3x+4}{2}$

$3$

$6$

$-1$

$5$

$\begin {cases}{x+y=190}\\{2×8x=22y}\end {cases}$

$\begin {cases}{x+y=-1}\\{x-y=3}\end {cases}$

D

$解:\left\{\begin{array}{l}3 x+2 y=2 k(1) \\5 x+4 y=k+3(2)\end{array}\right.$

$(2) -2 (1)得5 x+4 y-6 x-4 y=k+3-4 k$

$解得\ x=3 k-3\ $

$将\ x=3 k-3\ 代入 (1)得$

$9 k-9+2 y=2 k$

$解得\ y=\frac{9-7 k}{2}\ $

$x+y=3 k-3+\frac{9-7 k}{2}\ $

$\because x+y=-5\ $

$\therefore 3 k-3+\frac{9-7 k}{2}=-5$

$解得\ k=13\ $

$ $

$\left\{\begin{array} { l } { x = 5 , } \\{ y = 4 ; }\end{array} \left\{\begin{array}{l}x=10, \\y=2 .\end{array}\right.\right. $

$\begin {cases}{x=5}\\{y=10}\end {cases}$

$解:把(2)代入 (1)得$

$7 x+3(20-2 x)=100 \text {, }$

$解得\ x=40 , 把\ x=40\ 代入(2)得$

$y=-60 \text {, }$

$方程组的解是\ \left\{\begin{array}{l}x=40 \\ y=-60\end{array}\right. ;$

$ $

$解:(1) \times 2-(2) \times 3 \text { 得 } 2 t=49\ $

$t=24.5, \text { 代入(2)得 } 98-2 s+17=0\ $

$解得\ s=57.5\ $

$所以方程组的解是\ \left\{\begin{array}{l}s=57.5\\ t=24.5\end{array}\right.\ \ $