Question

The sum of the first 21 terms of the progression,-18-15-12 is

Answer 1

The given progression is an arithmetic progression with a common difference of 3. The formula to find the sum of the first n terms of an arithmetic progression is:

Sn = (n/2)(2a + (n-1)d)

Where Sn is the sum of the first n terms, a is the first term, and d is the common difference.

In this case, the first term (a) is -18 and the common difference (d) is 3. We want to find the sum of the first 21 terms (n = 21). Plugging these values into the formula, we get:

Sn = (21/2)(2(-18) + (21-1)(3))
= (21/2)(-36 + 20(3))
= (21/2)(-36 + 60)
= (21/2)(24)
= 21(12)
= 252

Therefore, the sum of the first 21 terms of the progression -18, -15, -12 is 252.