Question

Find the sum `1/15, 1/12, 1/10`, ............... to 11 terms.

Answer 1

Here, $$a = \frac{1}{15}, \ d = \frac{1}{60}, \ n = 11$$. 

$$S_{11} = \frac{11}{2}\left[2\left(\frac{1}{15}\right) + 10\left(\frac{1}{60}\right)\right]$$

$$= \frac{11}{2}\left(\frac{2}{15} + \frac{1}{6}\right)$$

$$= \frac{11}{2} \times \frac{9}{30}$$

$$= \frac{33}{20}$$