Clearly how it lies in 1st quadrant ... I didn't understand it.

Dear Student, 
Solution) In first quadrant, 
Value of x and y are positive. Also, x and y > 0.
Now, here $\alpha =\frac{\pi }{10}=\frac{{180}^0}{10}={18}^0$
So, its clear that,  the angles from 0⁰ to 90⁰ lies in the first quadrant. In the first quadrant, the values for sin, cos and tan are positive.
Hence, $$\begin{gathered} \tan \alpha lies in the first quadrant. \\ Regards! \end{gathered}$$
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