Finding a Cofunction with the Same Value as the Given Expression

Write $\tan \frac{π}{9}$ in terms of its cofunction.

Work it out on paper first. Then open the solution one step at a time, and stop as soon as you can finish on your own.

Solution
The cofunction of $\tan θ = \cot (\frac{π}{2} - θ) .$ Thus,

$\begin{matrix} \tan (\frac{π}{9}) & = & \cot (\frac{π}{2} - \frac{π}{9}) \\ = & \cot (\frac{9 π}{18} - \frac{2 π}{18}) \\ = & \cot (\frac{7 π}{18}) \end{matrix}$

Your turn

The book's Try It right after this example: same idea, new numbers. Only the answer is given.

Write $\sin \frac{π}{7}$ in terms of its cofunction.

How did it go?