Test Index

NCERT Class XI Mathematics - Sequences and Series - Solutions

© examsnet.com
Question : 49
Total: 106
Given a G.P. with a = 729 and 7th term 64, determine S7.
Solution:  
Let a be the first term and the common ratio be r.
We have a = 729, a7 = 64 ⇒ ar6 = 64
⇒ 729r6 = 64 ⇒ r6 =
64
729
 ⇒ r6 = (
2
3
)6 ⇒ r =
2
3
∴ S7 =
a(1−r7)
1−r
 = 729
[1−(
2
3
)7]
1−
2
3
 = 729
[1−
128
2187
]
1
3
 = 2187 [
2059
2187
] = 2059
© examsnet.com
Go to Question:
Prev Question
Next Question