Question

Find the sum of the following ap .1) 1/15, 1/12, 1/10,....,to 11 terms​

Answer 1

33/20

Explanation:

1/12 - 1/15 = 5/60 - 4/60 = 1/60

d = 1/60

a_n = a_1 + d(n - 1)

a_11 = 1/15 + (1/60)(11 - 1)

a_11 = 1/15 + 1/6

a_11 = 4/60 + 10/60

a_11 = 14/60

a_11 = 7/30

a_12 = 14/60 + 1/60

a_12 = 15/60

a_12 = 1/4

s_n = n(a_1 + a_n)/2

s_11 = 11(1/15 + 7/30)/2

s_11 = 11(2/30 + 7/30)/2

s_11 = 11(9/30)/2

s_11 = 99/60

s_11 = 33/20