Rewrite cot (81°) in terms of its co function.
Each trigonometric function has a co-function. The sine’s co-function is the cosine, the tangent’s co-function is the cotangent and the secant’s co-function is the cosecant. Thus, the abbreviation of the co-function of cot θ is tan θ.
According to the co-function identities, the trigonometric function of any angle is equal to the co-function of the angle’s complement.
Therefore, cot (81°) = tan (9)°.