Question

If the 21st term of an A.P is 25.find the sum of its first 41st 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 :)

Answer 2

a+(21-1)d=25
a+20d=25
sum =n÷2{2a+(41-1)d}
n÷2{2(a+20d)}
n÷2{50}
(41÷2)×50= 20.5×50=1025