1.

Let `a_1,a_2,a_3….., a_(101)` are in G.P with `a_(101) =25 ` and `Sigma_(i=1)^(201) a_i=625` Then the value of `Sigma_(i=1)^(201) 1/a_i` eaquals _______.

Answer» Correct Answer - 1
Let a be the first fterm and r be the common ratio of G.P. Then
`a(1-r^(201))/(1-r)=625` (1)
Now `sum_(r=1)^(201)1/(a_(i))=1/(a_(1))+1/(a_(2))+..+1/(a_(201))`
`=1/a+1/(ar)+..+1/(ar^(200))`
`=(1/a((1/r)^(201)-1))/((1/r-1))`
`=1/a((1-r^(201))/(1-r))1/r^(200)`
`=1/axx625/axx1/r^(200)` [from (1)]
`=625/((ar^(100))^(2))`
`=625/(a_(101))^(2)`
`=625/625=1`


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