Here radian is not mentioned
So, why donot we do it in degree?
The Reason for this question lies in the geometric interpretation of $\lim_{\Theta \rightarrow 0} {}\frac{Sin\Theta }{\Theta } = 1$
In calculus , $\frac{d}{dx} (sinx)) = cosx$ (or) $\lim_{x\rightarrow 0} {}\frac{sinx}{x} = 1$ is valid if x is in radian.
So, Differentiation/Integration of all trigonometric and inverse trigonometric functions is valid if x is in radian.
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Geometric Interpretation of $\lim_{\Theta \rightarrow 0} {}\frac{Sin\Theta }{\Theta } = 1$ :-
Suppose, We have a circle with radius 'r' = 1 unit