### Theory:

The equation is a statement of equality that contains one or more unknown value or variables.
Example:
$\begin{array}{l}\mathit{The}\phantom{\rule{0.147em}{0ex}}\mathit{following}\phantom{\rule{0.147em}{0ex}}\mathit{are}\phantom{\rule{0.147em}{0ex}}\mathit{equations}\phantom{\rule{0.147em}{0ex}}\mathit{with}\phantom{\rule{0.147em}{0ex}}1\phantom{\rule{0.147em}{0ex}}\mathit{variable}:\phantom{\rule{0.147em}{0ex}}\\ \\ 9x+10=6,\\ \\ {y}^{2}+6=10,\\ \\ \frac{6x}{10}-\frac{9}{10}=6\end{array}$

$\begin{array}{l}\mathit{The}\phantom{\rule{0.147em}{0ex}}\mathit{following}\phantom{\rule{0.147em}{0ex}}\mathit{are}\phantom{\rule{0.147em}{0ex}}\mathit{equations}\phantom{\rule{0.147em}{0ex}}\mathit{with}\phantom{\rule{0.147em}{0ex}}2\phantom{\rule{0.147em}{0ex}}\mathit{variable}:\phantom{\rule{0.147em}{0ex}}\\ \\ 9x+10y=6,\\ \\ {y}^{2}+6x=10,\\ \\ \frac{6x}{10}-\frac{9y}{10}=6\end{array}$
An equation is always equated to either a numerical value or another algebraic expression.

Now we understood what an equation is. Then we will see about linear equation.
An equation involving only linear polynomial is called a simple equation or a simple linear equation.
The highest power of the variable in the linear equation should be one. If the power of the variable is more than one that is not considered as a linear equation.

$\mathit{In}\phantom{\rule{0.147em}{0ex}}\mathit{linear}\phantom{\rule{0.147em}{0ex}}\mathit{equation}\phantom{\rule{0.147em}{0ex}}\mathit{the}\phantom{\rule{0.147em}{0ex}}\mathit{highest}\phantom{\rule{0.147em}{0ex}}\mathit{power}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{the}\phantom{\rule{0.147em}{0ex}}\mathit{variable}\phantom{\rule{0.147em}{0ex}}=1$
Example:
$\begin{array}{l}\mathit{The}\phantom{\rule{0.147em}{0ex}}\mathit{following}\phantom{\rule{0.147em}{0ex}}\mathit{are}\phantom{\rule{0.147em}{0ex}}\mathit{linear}\phantom{\rule{0.147em}{0ex}}\mathit{equations}:\\ \\ 9x+10=6,\\ \\ 6x=10,\\ \\ \frac{6x}{10}-\frac{9}{10}=6\end{array}$

$\begin{array}{l}\mathit{The}\phantom{\rule{0.147em}{0ex}}\mathit{following}\phantom{\rule{0.147em}{0ex}}\mathit{are}\phantom{\rule{0.147em}{0ex}}\mathit{not}\phantom{\rule{0.147em}{0ex}}\mathit{linear}\phantom{\rule{0.147em}{0ex}}\mathit{equations}:\\ \\ 9{x}^{2}+10y=6,\\ \\ {y}^{2}+6x=10,\\ \\ \frac{6{y}^{2}}{10}+\frac{10}{9}y=9.\end{array}$
In upcoming exercises, we will deal with equations with linear expressions in one variable only. Such equations are known as linear equations in one variable.