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$\sec(\theta)=x/5$.

What does $\sin(\theta)$ equal? What does $\tan(\theta)$ equal?

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    [Here is my answer to a similar, though not identical, problem](http://math.stackexchange.com/questions/15514/how-can-i-find-the-derivative-of-y-sin-arctan-x-tan-arcsin-x/15517#15517).2011-04-29

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As Michael Chen has alluded to:

Start by noting that

$\sec \theta = \frac{1}{\cos \theta} = \frac{x}{5}$

so that $\cos \theta = \frac{5}{x}$

Now you know that $\cos \theta$ is the 'adjacent / hypotenuse ' of a right angled triangle. So...draw the triangle, and work out what the unknown side ('opposite') will be, and then use the definition of $\tan$ and $\sin$.

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    @sara: To "work out... the unknown side", use the Pythagorean Theorem with x as the length of the hyponetuse.2011-04-29