Sn=21+61+121+201….N terms S2025=n=1∑2025n(n+1)1=n=1∑2025(n1−n+11)=(11−21)+(21−31)…⋯(20251−20261) $\begin{aligned}
& =\frac{2025}{2026} \
& \sqrt{2026 . \mathrm{S}{2025}}=\sqrt{2025}=45
\end{aligned}Given:\frac{6}{2}[-2 p+(6-1) p]=45\begin{aligned}
& 9 \mathrm{p}=45 \
& \mathrm{p}=5 \
& \left|\mathrm{~A}{20}-\mathrm{A}_{15}\right|=|-5+19 \times 5|-[-5+14 \times 5] \
& =|90-65| \
& =25
\end{aligned}$