8.04 Arithmetic series (Enrichment)

Find the sum of the first $10$10 terms of the arithmetic sequence defined by $u_1=-9$u1​=−9 and $d=2$d=2.

Evaluate the following sum up to the first $13$13 terms:

$-20-18-16$−20−18−16 $-$− ...

Find the sum of the first $19$19 terms in the arithmetic sequence $33+30+27$33+30+27 $+$+ ...