NCERT Solutions
Class 11 Maths
Sequences and Series

Ex.Misc.Q.21
Find the sum of the following series up to n terms:
(1) 5 + 55 + 555 + …….
(2) .6 +.66 +. 666 +………….
(1) The given series is 5 + 55 + 555 + ……...
Now, Sn = 5 + 55 + 555 + ……… to n terms
Sn = 5[1 + 11 + 111 + ……… to n terms]
Sn = (5 ÷ 9) [9 + 99 + 999 + ……… to n terms]
Sn = (5 ÷ 9) [(10 - 1) + (102 - 1) + (103 - 1) + ……… to n terms]
Sn = (5 ÷ 9) [(10 + 102 + 103 + ……… to n terms) – (1 + 1 + 1 + …. to n terms)]
Sn = (5 ÷ 9) [(10(10n - 1) ÷ (10 - 1) – n]
Sn = (5 ÷ 9) [(10(10n - 1) ÷ 9 – n]
Sn = {50(10n - 1) ÷ 81} – {5n ÷ 9}
(2) The given series is .6 +.66 +. 666 +………….
Now, Sn = .6 +.66 +. 666 +………….… to n terms
Sn = 6[0.1 + 0.11 + 0.111 + ……… to n terms]
Sn = (6 ÷ 9) [0.9 + 0.99 + 0.999 + ……… to n terms]
Sn = (6 ÷ 9) [(1 – (1 ÷ 10)) + (1 – (1 ÷ 102)) + (1 – (1 ÷ 103)) + ……… to n terms]
=> Sn = (6 ÷ 9) [ (1 + 1 + 1 + …. to n terms) - ((1 ÷ 10) + (1 ÷ 102) + (1 ÷ 103) + ……… to n terms)]
Sn = (2 ÷ 3) [n - (1 ÷ 10) {(1 – (1 ÷ 10) n) ÷ (1 – (1 ÷ 10))]
Sn = (2n ÷ 3) – (2 ÷ 30) × (10 ÷ 9) × (1 – 10-n)
Sn = (2n ÷ 3) – (2 ÷ 27) × (1 – 10-n)