Question

The steps for deriving the Quadratic formular are shown. Which best choose for the missing reason?

Answer 1

In the part inside the yellow circle, we multiplied by 1 the term -c/a, that is

$-\frac{c}{a}=-\frac{c}{a}\times1=-\frac{c}{a}(\frac{4a}{4a})=-\frac{4ac}{4a^2}$

where 1 was written as 4a/4a.

Since we must add this result to

$\frac{b^2}{4a^2}$

the answer is option C, change to LCD ( Least Common Denominator) because both terms must have the same denominator, which is 4a^2.