Find:a)the tenth term,and b) the sum of the first 21 terms of the progression -10,-8,6,.....

Answer 1

If you have a typo, and the sequence starts -10, -8, -6, then we clearly have
a = -10
d = 2
So, as with all AP's, the nth term is a + 2(n-1)
the sum of the first n terms is n/2 (2a + (n-1)*d)
so plug in your numbers.

Answer 2

Solved for a

Answer 3

210