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Completing the square
A method of rewriting a quadratic into a perfect-square form (plus or minus a constant), allowing solutions by taking square roots.
Discriminant (Δ)
The value $\Delta=b^2-4ac$ for $ax^2+bx+c=0$, which determines the number of real solutions.
Quadratic equation
An equation that can be written in the form $ax^2+bx+c=0$ where $a\neq 0$.