trigonometry - Why is the cosine of a right angle, 90 degrees, equal to zero? - Mathematics Stack Exchange

Why the cosine of an angle of 90 degree is equal to zero? By definition we know that: $$\text{cos } \alpha = \frac{\text{adjacent}}{\text{hypotenuse}}.$$ If we want to apply the definition to the

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trigonometry - Why is the hypotenuse in trig always positive regardless of the quadrant? - Mathematics Stack Exchange

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