For summing products of consecutive terms of an AP: $T_r$ = $a_r$ * $a_{r+1}$ * ... * $a_{r+p-1}$. Define $V_r$ = $a_r$ * $a_{r+1}$ * ... * $a_{r+p}$. Then $V_{r+1}$ - $V_r$ = (p+1)d$T_r$ (where d is common difference). So sum($T_r$) = [$V_{n+1}$ - $V_1$]/[(p+1)*d]. Example: sum(r(r+1)) = [r(r+1)$\frac{r+2}{3}$] evaluated at boundaries.