How come point B make (cos $\alpha$ , sin $\alpha$) for a unit circle
If it was in the first quadrant i could figure it out.But since it is in the second quadrant how come it happen.You cant directly go for a right angle triangle.
Angle $\angle BAD$ is making 180-$\alpha$ degree ..But i cant figure it out the more useful facts on how its making cos $\alpha$, and sin $\alpha$ as coordinates of point B.
I am trying to understand the proof in page 16 ..but became clueless as it is taken for granted
https://ncert.nic.in/textbook/pdf/kemh103.pdf
Please help me...If this is a stupid question ..please have some kindness towards me. n