Explanation

(b)
Multiplying the numerator and denominator by $\omega$ and $\omega^{2}$ respectively of I and II expression, we get
$\frac{\omega(a + b\omega + c\omega^{2})}{b\omega + c\omega^{2} + a} + \frac{\omega^{2}(a + b\omega + c\omega^{2})}{c\omega^{2} + a + b\omega}$
$= \omega + \omega^{2} = - 1\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \lbrack\because 1 + \omega + \omega^{2} = 0\rbrack$