Hard
Find the sum of the first 18 terms of the AP with first term 4 and common difference -2.
Explanation
Explanation
The sum of the first n terms of an AP is Sₙ = n/2 × [2a + (n-1)d]. Here n = 18, a = 4, d = -2. So S = 18/2 × [2(4) + (18-1)(-2)] = 18/2 × [8 + -34] = 18/2 × -26 = -234.
PRO TIP
Re-check your working by substituting the answer back into the original question.