TI 89 Programming r(θ)

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.
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This will take a fair amount of code. In a nutshell, you need a function that takes two inputs: a function and an input to that function.

Your function will need to have logic to determine what the user's input function is, and then evaluate that function with the input value.

Your function should be able to accept input functions to do the arithmetic operations (add, subtract, multiply, divide), as well as the trig functions that you want it to work with, plus whatever functions you think you need.

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