I was reading this article ...

http://www.goiit.com/posts/list/community-shelf-maths-trick-no-2-functions-question-1065320.htm

The method describe in the question is ... not always applicable .. this question is easily solved ..when you apply a little bit brain ..... please avoid tricks in every question ....

```` Given f(x+y) = f(x).f(y) => this is possible only if f(x) = a^x ...isn't ?

Because a^(x+y) = a^x.a^y  => f(x+y) = f(x).f(y) is possible in this case only ...a is variable ...it can be 5 , 6 ,7 ,3 or e ...

For this decision ...in question given f(1) = 3 ..

so f(x) = a^x => f(1) = a = 3

Means f(x) = 3^x ...

Now ...sigma ( r=1 to r=n )  3^r  =  ?

so it will convert to =>  3 + 3^2 + 3^3 + 3^4 ......+3^n  =  3(3^n-1) / 2   ans  ( simply sum of G.P)

Hope you guys understand what i am saying

 i agree with u sir....but i think its a gud thing if u can solve a question by more than 1 way.....these tricks helped me a lot in high pressure situations.....i respect ur view....everyone has different opinions..

Regards : Deepanshu