Find the sum of the first $10$10 terms of the arithmetic sequence defined by $a=6$a=6 and $d=3$d=3.

Find the sum of the first $10$10 terms of the arithmetic sequence defined by $a=-18$a=−18 and $d=4$d=4.

Find the sum of the first $15$15 terms in the arithmetic sequence $2+6+10$2+6+10 $+$+ ...

Evaluate the following sum up to the first $15$15 terms:

$-17-15-13$−17−15−13 $-$− ...