Question

for medal: Is the "u" in double angle formulas, such as sin(2u)=2sin(u)cos(u), a function, variable, or constant? What is it?

Answer 1

Any expression that can be evaluated as an angle.

Answer 2

Or, a variable that has a meaningful domain for the sin and cos functions.

Answer 3

in the expression you have given though, it is a variable. However, if you replaced the u with a 1, it would simply be a constant, given the usage of universal instantiation.