Answer
a) $21$
b) $3500$
c) $2620$
Work Step by Step
Use formula: $\Sigma_{k=1}^n k=\dfrac{n(n+1)}{2}$ and $\Sigma_{k=1}^n k^2=\dfrac{n(n+1)(n+2)}{6}$
a) Here, we have $\Sigma_{k=1}^{7} (3)=(3)(7)=21$
b) Here, we have $\Sigma_{k=1}^{500}(7) =(500)(7)=3500$
c) Here, we have $\Sigma_{k=3}^{264} 10=\Sigma_{j=1}^{262} 10=(262)(10)=2620$