A quadratic equation or second degree equation in $x$ is written in standard form as
$a{x^2} + bx + c = 0$ with $a \ne 0$

If $a = 0$, then the quadratic equation reduces to linear equation, i.e., $bx + c = 0$.

The word quadratic is derived from the Latin word quadratum which means “related to two or to make square”.

Examples:

1. $2x + 6{x^2} + 1 = 0$
2. $3{x^2} = 12$
3. $2{x^2} – 1 = 5x + 9$