Question

The sum of first n terms of an AP is (4n - n
^2
). Find the sum of first five terms.​

Answer 1

Put n=5

4x5-5^2
20-25
-5



PLEASE MARK IT BRANLEIST

Answer 2

Answers

We have been given ,

Sn = 4n -n^2

Just Think The Given Equation Of The Sum Of " n " Terms Is correct for all the terms in the AP , So If you put n=1 You get Sum of 1 terms which is actually the value of first term a1 .

It goes like this ,

Sn = 4n - n^2

= 4(1) - (1)^2

= 4 - 1 = 3

so The First Term Is 3.

Sum Of 2 Terms Is ...

S2 = 4(2) - (2)^2 = 4

Sum of first 2 Terms means a1 + a2 ,

Now you know a1 and S2 , so S2 - a1 = a2 = 4-3= 1

Now the common difference ,

a2- a1 = d = 1-3 = -2.

Hence The Basic Method Of Doing That was this .

Solving for a10 , an , a3 we get ,

a3 = -1 ;

a10 = -15;

an = 5 - 2n ;