Test. \[ \left( \sum_{k=1}^n a_k b_k \right)^2 \] Blah \[ \left|\sum_{i=1}^n a_ib_i\right| \le \left(\sum_{i=1}^n a_i^2\right)^{1/2} \left(\sum_{i=1}^n b_i^2\right)^{1/2} \]