sum taken two at a time is S..

 

now S = { S_M  +  sum of individual squares of g.p..} / 2

 

now wat is this individual square sum..

actually it  is a g.p with first term a² and common ratio r²  ........

 

so tht sum is equal to..a² { a^2m - 1} / r² -1...

 

write this in term of S_M  and get the ans..