Which one of the following is NOT a correct identity?

The correct double-angle tangent has 1 − ta$n^2$θ downstairs; 2tanθ/(1 + ta$n^2$θ) is sin2θ, so the two formulas have been crossed. Test at θ = 23°: 2tan23°/(1 + ta$n^2$23°) = 0.719, which is sin46°, while tan46° = 1.036. The other three are standard, including cot2θ = (co$t^2$θ − 1)/(2cotθ), which at θ = 23° gives 0.966 = cot46°.