Trigonometric Ratios and Converse of Pythagoras Theorem Lemma

Question

I am not quite sure what the question is asking here really. I understand the Lemma. Do I prove that $a^2+b^2=1$, using the given expressions for $a$ and $b$?

Thank you!

Answer 1

Yes, you need to prove that $a^2+b^2=1$, using the given expressions for $a$ and $b$. You also need to show that $a$ and $b$ are positive (you'll see then why you need $\alpha < 45^\circ$).

Then, by applying the Lemma you get the conclusion of the exercise.