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I am trying to prove that two functions are equivalent and I am required to use the function $g(x)=\cos{\frac{x}{55}}$ to prove that $h(x)=55\left(\frac{\pi}{2}+\arctan{(\sinh(4x)}\right)$ is equivalent to $f(x)=110\arctan{(e^{4x})}$, by considering $g(h(x))$ and $g(f(x))$. I have completed the algebra and have shown that $g(h(x))=g(f(x))$, but the question also asks which property of $g$ I have used for the proof. Any help is appreciated.

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    – cigien
    May 5 at 4:17
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This is called injectivity; if $g(a) = g(b)$ implies $a=b,$ then we say $g$ is injective. Note that cosine is not injective over its entire domain, so be careful with how your proof works.

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