Equivalence of ratios: $$a : b = ak : bk$$, where $$k$$ is a constant.
$$a,b,c,d$$ four terms are in proportion.
Invertendo: $$a:b=c:d$$ then $$b:a=d:c$$
Alternendo: $$a:b=c:d$$ then $$a:c=b:d$$
Componendo: $$a:b=c:d$$ then $$\dfrac{a+b}{b}=\dfrac{c+d}{d}$$
Dividendo: $$a:b=c:d$$ then $$\frac{a-b}{b}=\frac{c-d}{d}$$
Componendo-Dividendo: $$a:b=c:d$$ then $$\dfrac{a+b}{a-b}=\dfrac{c+d}{c-d}$$