inline: \left( \sum_{k=1}^n a_k b_k \right)^{\!\!2}
display:
$$ \left( \sum_{k=1}^n a_k b_k \right)^{\!\!2} \le \left( \sum_{k=1}^n a_k^2 \right) \left( \sum_{k=1}^n b_k^2 \right) $$