MATLAB: Differentiate with respect to ‘cos(theta)’

diff()differential equations

Hi, I am trying to differentiate with respect to cos(theta), but I can't seem to get matlab to do this. If I use the diff function I get:
>> diff(cos(the),cos(the))
ans =
diff(cos(the), cos(the))
When the answer I'm looking for is 1. If anyone knows of a way to do this it would be greatly appreciated! Cheers!
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