integer math. 9/5 is 1
5/9 is zero
just add a decimal:
also please use code tags. <> on the editor or code & /code in  brackets
its <cmath> not math.h (math.h is for C programs)
c is not initialized in at least some of your functions.
useless variables are useless. harmless, but clutter etc.
1 2 3 4 5 6 7 8 9
double CtoF (double degf); //a, double b) //what are a and b, and why not use them somewhere? and no ; on this line!!!
cout<<fixed<<setprecision(2); //this does not belong here. it belongs near where you print.
double c, f; //what is C??
return 1.8 * degf +32; //?? what is C?
f = 9/5 * c + 32;return f;
PLEASE ALWAYS USE CODE TAGS (the <> formatting button) when posting code.
It makes it easier to read your code and also easier to respond to your post. http://www.cplusplus.com/articles/jEywvCM9/
Hint: You can edit your post, highlight your code and press the <> formatting button.
Secondly, but more repeatedly in your coding, you are ignoring integer division. The first mathematical operation done in that line is
5 / 9
Since 5 and 9 are both int constants, the result is an int constant. The result will be truncated to the next integer below for positive results, so it will yield 0. If you wrote the ostensibly more correct 9/5 it would still truncate down to 1.
Either just add a decimal point to one of the numbers (e.g. 5.0 instead of 5) or, once you have written the fraction the correct way up, use the end result 1.8.
I am well aware that you get away with this in Python 3, but C++, in common with most other languages, does integer division when the operands are integers.
5/9 and 9/5 use integer arithmetic - so the result is an integer (0 and 1 respectively). To get the division to return a real result, one (or both) of the dividend and divisor needs to be of type real. Hence using 9.0 instead of 9. 9.0 is a real number so the result is real.