Find the sum of 21 members of the arithmetic progression -15, -10, -5 …

Let us recall the formula for the sum of the n-th terms of an arithmetic progression (AP):
Sn = (2 * a1 + d (n-1)) / 2 * n, where Sn is the sum of the nth AP members, a1 is the first AP member, n is the AP member number (in our case, it is 21), and d is AP difference
Let’s find the difference:
d = a2 – a1
d = -10 – (-15) = 5
We substitute the data into the formula:
Sn = (2 * (- 15) + 5 * (21-1)) / 2 * 21 = 735
Answer: 735