# Prove that $\cos^2\theta+\sin^2\theta=1$ [duplicate]

I try to find the question but I didn't

How do you do it? I'm really stuck on this proof. Can someone please explain?

• What definition of cos and sin are you using? – evil999man Apr 19 '14 at 10:07

Try geometrically: draw a unitary circle and an angle $\alpha$. Then $\sin\alpha$ and $\cos\alpha$ and the radius will form exactly a right angled triangle, and the formula you're searching for will be exactly the Pitagora Theorem.