TI 89 Programming r(θ)

clandarkfire

Hi, I'm trying to fine-tune a function I wrote for the TI 89 Titanium. Basically, for part of my function, I need it to plug in θ (an inputted variable/argument) into an expression r(θ) - also a an inputted variable/argument - and yield a numerical result.

In other words, the input format is function(expression,θ). If I were to type in function(cos(θ), π/3), I want it to return 1/2.

The problem I'm encountering is that I can't make it treat a variable as an expression. I tried simply writing r(θ), hoping that it would yield the result that occurs when the number θ is put into the expression r, but then it interprets it as r*θ.

So, how can I make it plug θ into the expression?
Any help would be appreciated.