Question

Find the sum 9 + 7 + 5 + 3 ...... to 14 terms.

Answer 1

Here, $$a = 9,\ d = -2,\ n = 14$$. 

$$S_{14} = \frac{14}{2}[2(9) + 13(-2)]$$

$$= 7(18 - 26)$$

$$= 7(-8)$$

$$= -56$$