Question

The sum of few terms of any ratio series is 728, if common ratio is 3 and last term is 486, then first term of series will be.

• A. 2
• B. 1
• C. 3
• D. 4

Answer 1

Answer: (A)

2

Answer 2

term of series

$= a r ^ { n - 1 }$ $= a ( 3 ) ^ { n - 1 } = 486$ …..(i)

and sum of n terms of series.

$= 728 ( \because r > 1 )$ …..(ii)

From (i), $a \left( \frac { 3 ^ { n } } { 3 } \right) = 486$ or

From (ii), $a .3 ^ { n } - a = 728 \times 2$ or

$1458 - a = 1456$ ⇒ $a = 2$ .