weird C problem

[Dj YB]

Hello,

I have written a small program in C and I am using the math.h library using 
#include<math.h>
so far everything is good.
when using the function sqrt I get the next error:

$ cc a.c
/tmp/ccTaon4H.o: In function `main':
a.c:(.text+0x3e): undefined reference to `sqrt'

to be sure I checked man sqrt:

SYNOPSIS

#include <math.h>

double sqrt(double x);

I have no idea what is wrong, any suggestions are welcome.

Thanks,
YB.