Let $z_{1}, z_{2}, ..., z_{n}$ be nonzero complex numbers, with $z_{k}=p_{k}\exp(i\theta_{k})$, where $p_{k}$ is a positive real number and $\theta_{k}$ real. Can you help me prove that $\left | \sum_{k=1}^{n}z_{k} \right |^2=\sum_{k=1}^{n}(p_{k})^2+2\sum_{k
Thank you