Use the rules of inequalities to prove the following inequalities.
If $a, b, c,$ and $d$ are positive numbers such that $\frac{a}{b}<\frac{c}{d},$ then $\frac{a}{b}<\frac{a+c}{b+d}<\frac{c}{d} \cdot[$ Hint: Show that $\left.\frac{a d}{b}+a<c+a \text { and } a+c<\frac{c b}{d}+c .\right]$