Question

Find the sum (–11) + (–7) + (–3) + 1 + ........ to 12 terms.

Answer 1

Here, $$a = -11, \ d = 4,\ n = 12$$. 

$$S_{12} = \frac{12}{2}[2(-11) + 11(4)]$$

$$= 6(-22 + 44)$$

$$= 6(22)$$

$$= 132$$