Fibonacci series - a different way
You should change this double x=0;double y=1; add newline after first ;
Using ++i is a best practice no matter if the type is int, so use ++i instead i++ and learn difference between them.
change main to int main, It is required to be standard compliant.
Why do you use doubles as x and y. Fibonacci numbers are always integers.
There are many fibonacci algorithms, you can look here 


https://stackoverflow.com/questions/14661633/finding-out-nth-fibonacci-number-for-very-large-n

http://fusharblog.com/solving-linear-recurrence-for-programming-contest/