Hard
Find the sum of the first 13 terms of the AP with first term -8 and common difference -1.
Explanation
Explanation
The sum of the first n terms of an AP is Sₙ = n/2 × [2a + (n-1)d]. Here n = 13, a = -8, d = -1. So S = 13/2 × [2(-8) + (13-1)(-1)] = 13/2 × [-16 + -12] = 13/2 × -28 = -182.
PRO TIP
Re-check your working by substituting the answer back into the original question.