hey this ques was solved a few days be4 also just look for it.... lemme tell u in brief actually the diff of diff in in gp wid common ratio 2.... general term in taht cas is a x r ^n + b x n + c put n as 1and r as 2 and equate it to 9 similarly wid n=2 equate it to 16... and wid again n=3 form 3 eqns get the value of a, b, c get the general term and thn u can sum it up.... if u hv any doubt u can nudge me... rate if useful