If $z_{1}=2+j, z_{2}=-2+j 4$ and $\frac{1}{z_{3}}=\frac{1}{z_{1}}+\frac{1}{z_{2}}$, evaluate $z_{3}$ in the form $a+j b$. If $z_{1}, z_{2}, z_{3}$ are represented on an Argand diagram by the points $\mathrm{P}, \mathrm{Q}, \mathrm{R}$, respectively, prove that $\mathrm{R}$ is the foot of the perpendicular from the origin on to the line PQ.