Question

the sum of first 9 terms of an ap is 351 and the sum of first 20 terms is 1770 find the 11th term

Answer 1

Dear Student,
Sum of first 9 terms= 9/2[2a+(9-1)d]

351 = 9/2[2a+8d]

39 = a+4d.........(i)

Sum of first 20 terms = 20/2[2a+19d]

1770 = 20[2a+19d]

1770/20 = 2a+19d

88.5 = 2a+19d..............(ii)

Solving eq i and ii

2a+19d=88.5

a+4d=39

d=21/22 and a = 387/11
11 term=a+10d
=387/11+21/22*10
=984/22