What is the derivative of $cos(-x)$ ?

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Answer 1 · 2016-06-09T01:15:52

$d/dxcos(-x) = -sin(x)$

Assuming we know that $d/dxcos(x) = -sin(x)$ , we could use the chain rule on $f@g(x)$ with $f(x) = cos(x)$ and $g(x) = -x$ .

However, a slightly nicer method is to note that cosine is an even function, meaning $cos(-x) = cos(x)$ . Thus

$d/dxcos(-x) = d/dxcos(x) = -sin(x)$

What is the derivative of cos(-x)cos(-x)?