Question

if 21th term of an ap is 25 ,find the sum of its first 41 term

Answer 1

Hey 

Given that :- 

a21 = 25 

=> a + 20d = 25 

=> a = 25 - 20d ––( i ) 

Now , 

Sum of it's first 41 terms :- 

= n / 2 [ 2a + ( n - 1 ) d ] 

= 41 / 2 [ 2 ( 25 - 20 d ) + ( 40 d ) ] 

= 41 / 2 [ 50 - 40 d + 40 d ] 

= 41 / 2 [ 50 ] 

= 41 / 2 * 50 

= 41 * 25 

= 1025 

thanks :)