Question

Find the sum of first 20 terms of the ap having sum of first 10 terms as52 and first 15 terms as 77

Answer 1

FIrst 10 terms sum = 52
First 15 terms sum=77
*NOw I will tell u a shortcut method
here ,*since it is an ap all terms differ by 'd'
*difference between 15 th term & 10th term is
77-52=25
so d = 25 /5 (since there are 5 terms between 15 & 5 )
d = 5
so add still 25 more to get the 20th term
i.e, 15th term + 25
i.e,77 + 25 =102
if u have any doubt pls ask