How do I do this problem ????
2 Answers

3/2+9/16+27/128+81/1024 + .....
Let S be the sum of the given series. Then
S = (3/2) + (9/16) + (27/128) + (81/1024) + .......... inf.
Here the common ratio (r) = Any term divided by the term that follows it.
For example choose the term (81/1024) the following term is (27/128) hence 
Common ratio (r) = (81/1024) divided by (27/128)
=> r = 3/8 This common ratio is less than one, hence the sum to infinite terms can be found out. ( Note that exact sum of a GP having infinite number of terms can be obtained iff, ' r ' is a fraction.)
Hence the required sum : 
S = [ a / ( 1  r ) ] , where a is first term of the series,
=> S = [ 3/2 divided by ( 1  3/8 ) ]
=> S = ( 3/2 ) divided by ( 5/8 )
=> S = 12/5 ..................................... Answer
PKT
Source(s): Its me only at <[email protected]> 
a = 3/2
r = 3/8
S = a(1  r^n)/(1  r)
= (3/2)[1  (3/8)^infinite]/[1  3/8]
(3/8)^infinite = 0 since the value becomes smaller and smaller as the power is increased.
So
S = a / (1  r) = 3/2 * 8/5 = 24/10
Hope this helps.