Question

Find the sum of the first ten terms of -10 -6 -2 + . . . .

Answer 1

n
sum = ---- [2a + (n-1)d]
2

In your case a = first term = -10
n = number of terms = 10
d = common difference = -6 -(-10) = -6 + 10 = 4
10
so, sum = ---- [2(-10) + (10-1)(4)] = 5[-20 + 36] = 5[16] = 80
2

so sum of first ten terms of the given arithmetic progression is 80