$$a^2+b^2=c^2 \implies \frac{a^2 + b^2}{c^2} = \left( \frac{a}{c} \right)^2 + \left( \frac{b}{c} \right)^2 = 1 \\ \implies \cos^2 \theta + \sin^2 \theta = 1.$$

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$$a^2+b^2=c^2 \implies \frac{a^2 + b^2}{c^2} = \left( \frac{a}{c} \right)^2 + \left( \frac{b}{c} \right)^2 = 1 \\ \implies \cos^2 \theta + \sin^2 \theta = 1.$$