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# Definition of an equation

An equation is an equality that involves one or several variables.

Here are several equations \begin{align*} &\Tred{x}-1 \Tblue{=} 0,\quad (\Tred{t}-1)(\Tred{t}-2)\Tblue{=}4,\quad \frac{\Tred{a}+1}{\Tred{a}^2-\Tred{a}} \Tblue{=} 2\\ &\qquad \Tred{x} + \Torange{y} \Tblue{=} 1,\quad \Tred{x}^2 + \Torange{y}^2 \Tblue{=} 1. \end{align*}

The variables in the equation are called the unknowns.

$\Tred{a}$ is an unknown in the equation $(\Tred{a}+1)(\Tred{a}-1) = 0$.

An equation is like a scale where two expressions need to be equal.