Answer
$f(x)=\sin x$ is an even function.
Work Step by Step
$$f(x)=\cos x$$
As we work out in exercise 23, $$\cos(-x)=\cos x$$
Therefore, $$f(-x)=\cos(-x)=\cos x=f(x)$$
According to the definition given, $f(x)=\sin x$ is an even function.
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