Question

Given Sn = 4n-n^2. Find the 11th term and also find the sum of first 7 terms...... Given - Sn = 4n-n^2 To find - a11 = ? S7 = ?

Answer 1

$\huge\boxed{Answer}$

Sn = 4n - n² ------(1)

So ,

Sum of first 7 terms -

S7 = 4(7) - 7²

= 28 - 49

S7 = -21

We know that

Sn = n/2 { a + An } ------(2)

From eq.(1) & eq.(2) -

n/2 {a + An} = n(4 - n)

1/2 {a + An} = (4 - n)

a + An = 8 - 2n

An = 8 - a - 2n

We n=1 , then

A1 = 8 - a - 2

{ A1 = first term = a }

a = 6 - a

2a = 6

a = 3

So , An = 5 - 2n

A11 = 5 - 2(11)

= 5 - 22

= -17