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Question

If a,a1,a2,a3,....,a2n,b are in AP and a,g1,g2,g3,....,g2n,b are in GP and h is the harmonic mean of a and b then a1+a2ng1g2n+a2+a2n1g2g2n1+....+an+an+1gngn+1 is equal to

A
2nh
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B
2nh
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C
nh
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D
nh
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Solution

The correct option is D 2nh
a1+a2ng1g2n+a2+a2n1g2g2n1+........+an+an+1gngn+1=n(2a+2ndar2n)
We got above expression by substituting a1,a2,....a2n,g1,g2,....g2n values.
a1=a+d,a2=a+2d,......,a2n=a+(2n1)d, where d is common ratio for AP.
g1=ar,g2=ar2,......,g2n=ar2n. where r is common ratio of GP
Therefore 2n(a+ndar2n)=2na+ba2ab=2na+b2ab=2n(a+b2ab)=2nh

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