1.

The sum of an infinite GP is `80/9` and its common ratio is `(-4)/5`. Find its first term.

Answer» Let a be the first term of the given infinite GP.
Here, `r=(-4)/5` and `|r|=|(-4)/5|=4/5 lt 1`.
The sum of this infinite GP is `S=80/9`.
`:. S=a/((1-r)) rArr a/((1+4/5))=80/9`
`rArr (5a)/9=80/9 rArr 5a=80 rArr a=16`.
Hence, the first term is 16.


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