Express the following in terms of trigonometric ratios of angles between 0° to 45° :

tan 81° + cos 72°

Solving,

⇒ tan 81° + cos 72°

⇒ tan (90° - 9°) + cos (90° - 18°)

As, tan(90° - θ) = cot θ, cos(90° - θ) = sin θ

⇒ cot 9° + sin 18°.

Hence, tan 81° + cos 72° = cot 9° + sin 18°.