There are a number of small functions throughout the course that are referred to in global namespace, for example:

• fmin, fmax
• sqrt
• fabs
• sin, cos, tan
• rand

Two of them -- sqrt and fabs -- are pulled into global namespace with the using directive inside rtweekend.h, but others -- aren't. Two possible solutions:

1. Prefix all functions with std:: namespace (preferred).
2. Pull missing functions into global namespace (poor practice).