## Trigonometry (11th Edition) Clone

$\tan$ is to be filled in the blank.
$$A72^\circ=\cot18^\circ\hspace{1cm}(1)$$ We notice that $$72^\circ+18^\circ=90^\circ$$ That means we can rewrite $(1)$ as $$A72^\circ=\cot(90-72^\circ)\hspace{1cm}(1)$$ Now from the Cofunction Identity: $$\tan\theta=\cot(90^\circ-\theta)$$ So $$A=\tan$$ In other words, we fill in the blank with $\tan$.