### A simple problem related to symbolic calculation

~~How ~~Could anyone let me know how you can ~~you defined ~~define a variable as some function of another variable without specific definition? For example, ~~I ~~how can you define theta as some function of x and then differentiate the 'sin(theta)' by x?

The following is my code that doesn't work. I couldn't find how to fix it in reference manuals. Any help will be appreciated.

var('theta, y, f')

y=sin(theta) ; theta=f(x);

y.derivative(x)