how to get the maths function work?

I used LPC1765 in the system, the program is as follows:

#include<math.h>

...

int main(void)

{ ...

float a,b,c;

...

b = 3.1;

a = log(b);

c = sin(a);

...

}

the program is compiled and linked with no error, but "a" and "c" can't get the correct value. who could help me?
thanks a lot!