cotx + tanx = cosx/sinx + sinx/cosx
= (cos2x + sin2x)/(sinxcosx)
= 1/(sinxcosx)
= 2/(2sinxcosx)
= 2/sin(2x)
= 2csc(2x)
Note: the equation that was given is not an identity. For example, when x = π/4, cotx+tanx = 2 but 2cscx = 2√2.