Completing the Square – Deriving the Perfect Square Form of a Quadratic

The Ultimate Crash Course for STEM Majors

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Complete the Square

$$x^2-\frac{1}{2}x-\frac{3}{16}=0,\qquad 16x^2-8x-3=0$$

Complete the Square

$$ax^2+b+c=0$$
$$a\left(x^2+\frac{b}{a}x+\frac{c}{a}\right)=0 \Rightarrow a\left(x^2+\frac{b}{a}x+\frac{c}{a}+0\right)=0$$
$$a\left(x^2+\frac{b}{a}x+\frac{c}{a} +\left[\left(\frac{b}{2a}\right)^2-\left(\frac{b}{2a}\right)^2\right]\right)=0$$
$$a\left(x^2+\frac{b}{a}x+\frac{c}{a} +\left(\frac{b}{2a}\right)^2-\left(\frac{b}{2a}\right)^2\right)=0$$
$$a\left(x^2+\frac{b}{a}x+\left(\frac{b}{2a}\right)^2 +\frac{c}{a}-\left(\frac{b}{2a}\right)^2\right)=0$$
$$a\left(x^2+\frac{b}{a}x+\left(\frac{b}{2a}\right)^2 +\frac{c}{a}-\left(\frac{b}{2a}\right)^2\right)=0$$

NOTE* Perfect square: $$(x+y)^2$$

Not perfect square: $$x^2+y^2$$

Factor: $$x^2-y^2=(x-1)(x+y)$$

$$ a\left( {\color{red}{x^2+\frac{b}{a}x+\left(\frac{b}{2a}\right)^2}} +\frac{c}{a} -\left(\frac{b}{2a}\right)^2 \right)=0 $$
$$ a\left( {\color{red}{xx}} + {\color{green}{\frac{1}{2}\frac{a}{b}x}} + {\color{green}{\frac{1}{2}\frac{a}{b}x}} + {\color{red}{\left(\frac{b}{2a}\right)\left(\frac{b}{2a}\right)}} +\frac{c}{a} -\left(\frac{b}{2a}\right)^2 \right)=0 $$
$$ a\left( {\color{magenta}{\left[x+\frac{b}{2a}\right]^2}} +\frac{c}{a} -\left(\frac{b}{2a}\right)^2 \right)=0 \Rightarrow a{\color{magenta}{\left[x+\frac{b}{2a}\right]^2}} + a\left[ \frac{c}{a} -\left(\frac{b}{2a}\right)^2 \right]=0 $$
$$ \Rightarrow a{\color{magenta}{\left[x+\frac{b}{2a}\right]^2}} = -a\left[ \frac{c}{a} -\left(\frac{b}{2a}\right)^2 \right] \Rightarrow {\color{magenta}{\left[x+\frac{b}{2a}\right]^2}} = -\frac{c}{a} +\left(\frac{b}{2a}\right)^2 = \left(\frac{b}{2a}\right)^2-\frac{c}{a} $$

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