• # question_answer $\sum\limits_{i=1}^{n}{{{a}_{i}}=0,}$ where $|{{a}_{i}}|\,=1,\,\,\forall \,i,$ then the value of $\sum\limits_{1\le i\le }{\,\sum\limits_{j<n}{{{a}_{i}}\cdot {{a}_{j}}}}$ is A)  $-\frac{n}{2}$                                  B)  $-n$ C)  $\frac{n}{2}$                                    D)  $n$

$T=\frac{2\pi }{\omega }$ $\Rightarrow$ $\xi =\frac{4BA\omega }{2\pi }=\frac{2BA\omega }{\pi }$ $I=\frac{P}{4\pi {{r}^{2}}}=\frac{60}{4\pi \times {{4}^{2}}}W/{{m}^{2}}$ ${{P}_{1}}=I\times \frac{\pi {{d}^{2}}}{4}$ ${{P}_{1}}=\frac{60}{4\pi \times {{4}^{2}}}\times \frac{\pi \times {{(2\times {{10}^{-3}})}^{2}}}{4}$ $=9.375\times {{10}^{-7}}\,J/s$