Cosine of a number with no math.h. Unknown error 1.#INF00

Cosine of a number with no math.h. Unknown error 1.#INF00

Well, I have a problems with a specific C code. I'm dealing with solving a
task without using any "refined" coding, that is, doing regular stuff with
the simplest of the libraries and functions. Particulary, I am ONLY
allowed to use stdio.h , not any library more. And no strings, also.
The problem: I have to do the cosine of a real number x (stored in
double), which is given by the user, using this expression: cos (x) =
((x^(2i))/((2i)!))*(-1)^i, all this inside a sumatore from i = 0 to i = n,
being n the precision (an int number) which the user chooses. Ok, I've
managed to do this code so far:
scanf("%lf", &x);
r1 = x;
scanf("%d", &n);
for (i = 0; i <= n; i++){
for(cont1 = 0; cont1 < 2*i; cont1++){
r1 = r1*r1;
}
for(cont2 = 1; cont2 <= 2*i; cont2++){
r2 = r2*cont2;
}
for(cont3 = 1; cont3 < i; cont3++){
r3 = (-1)*(-1);
}
result = result + (r1/r2)*r3;
}
printf("The result is %lf\n", result);
But the output is always 1.#INF00, all the time. Does anybody know why
this happens? Could somebody tell me how to manage to get the right code?
It would be very appreciated.
Thank you in advance.