[Kevin Sevcik]

math.h is just a header file. header files typically don't contain any code, they just have compiler directives in case that library needs other includes, etc. The actual code will be in a math.c file in a libraries folder or something. I'm afraid I can't tell you for certain because I'm not at a computer with the compiler at the moment. At any rate, either you're missing math.c or it's corrupted, or someone's fiddled with compiler settings and you aren't pointing the compiler to the proper library folder anymore.

EDIT:
From a brief perusal, I note that the include folder should be c:\mcc18\lib