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What is an equation?
A pair of expressions separated by an equal to sign is called as equation. This equal to sign indicates that the pair of expressions are equal.
Example:
$$5x = 20$$ is an equation. Here, $$x$$ is a variable, the expression $$5x$$ forms the left hand side of the equation (LHS), and the expression $$20$$ forms the right hand side of the equation (RHS).
Now, let us discuss about linear equation in one and two variables.

1.Linear equation in one variable:
Linear equation is an equation in which only one variable is involved and having the highest power as $$1$$. It is also known as simple equation.
Example:
$$5$$ times $$x$$ equals $$20$$.

Solution:

The algebraic expression is given by:

$$5x = 20$$

This equation has only one variable and having the highest power as $$1$$. This equation is a linear equation in one variable.
2. Linear equation in two variables:
Linear equation in which two variables are involved and having the highest power as $$1$$.
Example:
$$5x + 6y = 8$$, $$3x = 12 z$$