Question

FIND SUM OF FIRST 11 TERMS OF A.P 1/15,1/12,1/10.......

Answer 1

SOLUTION:-

Given:

A.P. is 1/15, 1/12, 1/10........

To find:

The sum of first 11 terms.

Explanation:

We know that, formula of the sum of A.P. is;

⇒$\frac{n}{2} [2a+(n-1)d]$

∴Here,

·n=11

·a=1/15

d=$\frac{1}{12} -\frac{1}{15} \\ \\ =\frac{5-4}{60} \\ \\ =\frac{1}{60}$

Putting the values above the formula:

$Sum=\frac{11}{2} [2*\frac{1}{15} +(11-1)\frac{1}{60} ]\\\\Sum=11(\frac{1}{15} +10*\frac{1}{60})\\\\Sum=11(\frac{1}{15} +\frac{10}{60} )\\\\Sum=11(\frac{4+10}{60} )\\\\Sum=11(\frac{14}{60} )\\\\Sum=\frac{154}{60} \\\\Sum=2.56$

Thus,

The sum of 11th term is 2.56.

Thank you.