207. Derive the formula for solving a quadratic equation, $a x^{2}+b x+c=0$, by completing the square.
Instruction. Multiplying the given equation by $4a$ and adding $b^{2}$ to both sides, transform it into the form:

$$
4 a^{2} x^{2}+4 a b x+b^{2}+4 a c=b^{2}
$$

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from which

$$
(2 a x+b)^{2}=b^{2}-4 a c
$$

or

$$
2 a x+b= \pm \sqrt{b^{2}-4 a c}
$$

or

$$
x=\frac{-b \pm \sqrt{b^{2}-4 a c}}{2 a}
$$