If you're seeing this message, it means we're having trouble loading external resources on our website.

# 方程组和代入法

$\begin{array}{rlr}x+y& =24& \text{方程2}\\ \\ x+2x& =24& \text{用2x 替换 y}\end{array}$

$\begin{array}{rlr}y& =2x& \text{方程 1}\\ \\ y& =2\left(8\right)& \text{用 8 替换 x}\\ \\ y& =16\end{array}$

$\begin{array}{rl}y& =2x\\ \\ 16& \stackrel{?}{=}2\left(8\right)& \text{把 x = 8 和 y = 16 代入}\\ \\ 16& =16& \text{对了！}\end{array}$

$\begin{array}{rl}x+y& =24\\ \\ 8+16& \stackrel{?}{=}24& \text{把 x = 8 和 y = 16 代入}\\ \\ 24& =24& \text{对了！}\end{array}$

$4x+y=28$
$y=3x$
$x=$
$y=$

## 找到其中一个变量的替换值，然后使用代入法.

$\begin{array}{rlr}-3x+y& =-9& \text{方程 1}\\ \\ -3x+y+3x& =-9+3x& \text{两边同时加 3x}\\ \\ y& =-9+3x& \end{array}$

$\begin{array}{rlr}5x+4y& =32& \text{方程2}\\ \\ 5x+4\left(-9+3x\right)& =32& \text{用 -9 + 3x 替换 y}\\ \\ 5x-36+12x& =32& \\ \\ 17x-36& =32& \\ \\ 17x& =68& \\ \\ x& =4& \text{两边同时除以 17}\end{array}$

$\begin{array}{rlr}-3x+y& =-9& \text{方程 1}\\ \\ -3\left(4\right)+y& =-9& \text{用 4 替换 x}\\ \\ -12+y& =-9& \\ \\ y& =3& \text{两边同时加12}\end{array}$

## 练一练!

1) 代入法解以下方程组.
$2x-3y=-5$
$y=x-1$
$x=$
$y=$

2) 代入法解以下方程组.
$-7x-2y=-13$
$x-2y=11$
$x=$
$y=$

3) 代入法解以下方程组.
$-3x-4y=2$
$-5=5x+5y$
$x=$
$y=$