The Pythagorean theorem $\text{hyp}^2 = \text{opp}^2 + \text{adj}^2$ leads directly to which trigonometric identity?

Dividing the Pythagorean relation $\text{hyp}^2 = \text{opp}^2 + \text{adj}^2$ by $\text{hyp}^2$ converts the sides into $\sin\theta$ and $\cos\theta$, yielding $\sin^2\theta + \cos^2\theta = 1$. This is why it is called a Pythagorean identity.