Find the sum of the first 20 terms of an arithmetic progression of which the third term is 55 and the last term is -98

The sum of first 20 arithmetic series

Given:

Arithmetic series for 3rd term is 55

Arithmetic series for 7th term is -98

To find:

The sum of first 20 Arithmetic series

Step by Step Explanation:

Solution:

Formula for calculating arithmetic series

Arithmetic series=a+(n-1) d

Arithmetic series for 3rd term

Arithmetic series for 19th term is

Subtracting equation 2 from 1

16d=-98-55

16d=-153

Also we know

First 20 terms of an AP  

Sum of 20 Arithmetic series is

Substitute the known values in the above equation we get

Result:

Thus the sum of first 20 terms in an arithmetic series is

the correct answer is 5

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step-by-step explanation: