# Where can I find difficult trigonometric formulas?

I understand simple trigonometric formulas like $$\sin^2 \alpha + \cos^2 \alpha = 1.$$ I've found ones in books but I've not found more difficult like $$\sin^2 \frac{\alpha}{2} = \frac{1 - \cos \alpha}{2}$$ or inverse trigonometric functions. Is there anywhere it?

• en.wikipedia.org/wiki/List_of_trigonometric_identities has a lot. If you want a more extensive list, I don't know. Sep 21, 2015 at 13:55
• there are only statement/identities but i seek parsing something like $$\sin^2 \alpha + \cos^2 \alpha = 1 => b^2 + a^2 = c^2 => \frac{b^2}{c^2} + \frac{a^2}{c^2} = 1$$ Sep 21, 2015 at 14:03

Once you get past the introductory right angle approach to trigonometry, and identities such as Pythagorean and complimentary angle identities, you should start seeing that "trig" triangles and arbitrary $$(a,b,c)$$ triangles as all similar.