if pth ,qth and rth terms of a G.P. are themselves in G.P.,show that p,q,r are in A.P.

AP= arp-1  , Aq = arq-1,Ar = arr-1

now arp-1 , arq-1  ,  arr-1    

are in gp

that means arq-1 / arp-1   = arr-1 / arq-1

  r q-1 / rp-1 = rr-1 / rq-1

  r q-1-p+1 = rr-1-q+!

  rq-p = rr-q

  i.e  q-p = r-q

  2q = p+r

 therefore p,q,r  are in AP

 

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