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Question

how to evaluate $\arcsin(x)-\arccos(x)=\arccos(\frac{\sqrt{3}}{2})$

Thoughts

Do you cos on both sides to finally do quadratic equation to solve at the end of the problem? so i got $0=4x^2+4x-1+2\sqrt{3}$ on my last step but i dont know if this is the right approach

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Note that $\arcsin(x)+\arccos(x) =\pi/2 $.

Therefore $\arcsin(x)-\arccos(x) =\arccos(\frac{\sqrt{3}}{2}) $ becomes $\arccos(\frac{\sqrt{3}}{2}) =\pi/2-2\arccos(x) $ or $\arccos(x) =(\pi/2-\arccos(\frac{\sqrt{3}}{2}))/2 $.

You should be able to take it from here.

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You can't take the cosine of both sides, at least not the way you probably did. You must do it as follows:

$$\cos(\arcsin(x)-\arccos(x))=\frac{\sqrt3}2$$

And then use sum of angles identity:

$$\cos(\arcsin(x))\cos(\arccos(x))+\sin(\arcsin(x))\sin(\arccos(x))\\=x\cos(\arcsin(x))+x\sin(\arccos(x))$$

And use pythagorean theorems (let's assume all positive)

$$=x\sqrt{1-\sin^2(\arcsin(x))}+x\sqrt{1-\cos^2(\arccos(x))}\\=2x\sqrt{1-x^2}$$

Can you take it from here?

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From the unit circle, $\displaystyle \arccos\frac{\sqrt3}2 = \frac\pi6$.

A known property relating inverse sine and inverse cosine is $\arcsin x + \arccos x = \dfrac\pi2$. Therefore:

\begin{align*} \arcsin x - \arccos x &= \arccos \frac{\sqrt3}2\\[0.3cm] \frac\pi2 - \arccos x - \arccos x &= \frac\pi6\\[0.3cm] -2\arccos x &= -\frac\pi3\\[0.3cm] \arccos x &= \frac\pi6 \end{align*}

Now take the cosine of both sides.

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    $\begingroup$ Same idea as mine, and you went all the way. $\endgroup$ – marty cohen Jan 20 '17 at 1:53
  • $\begingroup$ @martycohen, I could say likewise (except for the second part) as you posted yours less than 1 minute before mine. And I dunno about all the way, I left a step at the end there XD $\endgroup$ – tilper Jan 20 '17 at 19:18
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I would just solve this using the unit circle. If you're working in degree mode, $\arccos(\frac{\sqrt{3}}{2}) = 30^{\circ}$. So then, can you find a ratio $x$ such that $\arcsin(x) - \arccos(x) = 30^{\circ}$? If you look at a unit circle, you should see an immediate answer...

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