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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In trigonometry, what is an intuitive explanation of the law of cosines? - Quora

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trigonometry - Prove that the cosine distance between any two vectors of 0's and 1's of the same length is at most 90 degrees. - Mathematics Stack Exchange

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How to find values of non-standard angles like 46, 55, 98, 102 degrees in trigonometry without the help of a calculator and log book as both are not used in the exam - Quora

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I have to memorize over 30 Trigonometric identities and additional stuff. How can I do it? : r/learnmath

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