csc^2(x)=1/sin^2(x)
d/dx[csc^2(x)]=d/dx[1/sin^2(x)]
d/dx[1/sin^2(x)]=d/dx[[sin(x)]^{-2}]
let u=sinx
d/dx[[sin(x)]^{-2}]=d/{du}[u^{-2}]d/dx[sinx]
d/{du}[u^{-2}]= -2u^{-3}
d/dx[sinx] = cosx
d/dx[[sin(x)]^{-2}]=-2u^{-3}cosx=-{2cosx}/{sin^3x}
cosx/sinx=cotx => -{2cosx}/{sin^3x}=-{2cotx}/{sin^2x}
1/sin^2x=csc^2x => -2cotxcsc^2x
d/dx[csc^2(x)]= -2cotxcsc^2x