First slide

Special Series in Sequences and Series

Question

If the sum of the first 40 terms of the series, $3 + 4 + 8 + 9 + 13 + 14 + 18 + 19...........$ is (102)m, then m is equal to:

Moderate

A

20
B

25
C

10
D

5

Solution

$S = 3 + 4 + 8 + 9 + 13 + 14 + 18 + 19........40$ terms
$\Rightarrow S = 7 + 17 + 27 + 37 + 47 + .....20$ terms
$∴ S_{40} = \frac{20}{2} [2 \times 7 + (19) 10] = 10 [14 + 190] = 10 [2040] = (102) (20)$
$\Rightarrow m = 20$

Get Instant Solutions

When in doubt download our app. Now available Google Play Store- Doubts App

Recieve an sms with download link

Doubts App

OR

SCAN ME

Doubts App

Doubts App