What does it mean the notation $\int{R\left( \cos{x}, \sin{x} \right)\mathrm{d}x} $ Sometimes I find this notation and I get confused:
 $$\int{R\left( \cos{x}, \sin{x} \right)\mathrm{d}x} $$
Does it mean a rational function or taking rational operations between $\cos{x}$ and $\sin{x}$ ?
Can you explain please?
Update: I think you did not understand the question well,
Here is an example (maybe it is a lemma or a theorem):

All the integrals of the form $\int{R\left( \cos{x}, \sin{x}
 \right)\mathrm{d}x} $ can be evaluated using the substitution
  $u=\tan{\dfrac{x}{2}} $.

I think that $R$ here does not stand for a rational function but for taking rational operations(addition, subtraction, multiplication, division) between $\cos{x} $ and $\sin{x}$
Update : I did not noticed that $R$ is a rational function of two variables and that means exactly that we are taking rational operations.

Here $R$ is a function of two variables $s$ and $t$. For instance, if $$R(s,t) = \frac{s}{1+t}$$ then $$R(\cos x , \sin x) = \frac{\cos x}{1 + \sin x}.$$
