Question

the sum of first n terms of AP is 5 n - n square find the sum of first 20 terms of AP​

Answer 1

Answer :

- 300.

Given :

• Sn = 5n - n².

A/Q.

Sn = 5n - n²

=> a1 = S1 = 5(1) - (1)².

=> 5 - 1

=> 4.

_____________________________

S2 = 5(2) - (2)²

=> 10 - 4

=> 6.

______________

S2 - a1 = a2

=> a2 = 6 - 4

=> a2 = 2.

______________

d = a2 - a1

=> 2 - 4

=> - 2.

______________

S20 = 20/2 [ 2( 4 ) + ( 20 - 1 )(- 2 ) ]

=> 10 ( 8 - 38 )

=> - 300.