How and why did we extended the domain and definition of trigonometric function from the angles of right triangle to the angles of any triangle?

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Was it by the formula for $\sin(x+y)$, reference triangle or something? Which practical and mathematical problems required this extension?

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All the trig functions are ratios defined in terms of angles. We can extend the domain of these functions beyond pure right triangles using the unit circle. We needed to extend them first to deal with the Law of Cosines and The Law of Sines, which extends the Pythagorean Theorem to deal with obtuse and acute triangles. We also use trig functions to model periodic events, like the temperature throughout the year. Finally, we use trig functions throughtout Calculus for things like Fourier Transforms.