4. If $\frac{x-a-b}{c}+\frac{x-b-c}{a}+\frac{x-c-a}{b}=3$, where $a, b, c$ are positive constants, find $x$ in terms of $a, b$ and $c$.
(ans. $x=a+b+c$.
Substituting this into the equation, one gets $\frac{c}{c}+\frac{a}{a}+\frac{b}{b}=3$.)