Question

4. The sum of third and ninth term of an A.P is 8. Find the sum of the first 11 terms of the progression.

Answer 1

Hi ,

Let a and d are first term and common difference of an AP,

Third term = a + 2d

Ninth term = a + 8d

Sum = 8

a + 2d + a + 8d = 8

2a + 10d = 8 ----( 1 )


Sum of first and 11th terms

= a + a + 10d

= 2a + 10d

= 8 [ from ( 1 ) ]

I hope this helps you.

:)

Answer 2

a+2d+a+8d = 8
2a+10d = 8
s11 = 11/2(2a+10d)
= 11/2( 8 )
= 44