#f'(x)=frac{d}{dx}(e^{−x}sin(x)−e^xcos(x))#
#=frac{d}{dx}(e^{−x}sin(x))−frac{d}{dx}(e^xcos(x))#
#=[e^{−x}frac{d}{dx}(sin(x))+sin(x)frac{d}{dx}(e^{−x})]#
#−[e^xfrac{d}{dx}(cos(x))+cos(x)frac{d}{dx}(e^x)]#
#=[e^{−x}(cos(x))+sin(x)(-e^{−x})]#
#−[e^x(-sin(x))+cos(x)(e^x)]#
#=e^{−x}(cos(x)-sin(x))-e^x(cos(x)-sin(x))#
#=(e^{−x}-e^{x})(cos(x)-sin(x))#