Finding the alternate forms of ratios

Question

This is a super basic algebra question but I can't figure out how you get the alternate form of: $$\frac ab = \frac cd$$

Which is: $$\frac {a+b}b = \frac{c+d}d$$

The process explaining how we arrive to the alternate form

Answer 1

$1=\frac{b}{b}=\frac{d}{d}$. Add one to both sides of the equation $\frac{a}{b}=\frac{c}{d}$.