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I have a literal equation that needs to be solved for $\theta$:

$$mg \sin(\theta) = \mu mg \cos(\theta)\left({ M+m \over m}\right) $$

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    Do you mean $mg \sin \theta = umg \cos \theta \cdot \frac{M+m}{m}$?2012-08-28
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    Use $\sin x/\cos x=\tan x$ and the arctangent function.2012-08-28
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    @jay: Did I get it right? If so, you can use the "edit" button to see what I did to change the formatting. If not, say what's wrong and someone will fix it.2012-08-28
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    What is a «literal equation»?2012-08-28
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    @MarianoSuárez-Alvarez It's an equation whose coefficients are letters instead of numbers, I guess. Such as $ax+b=0$, but not $2x-5=0$.2012-08-28
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    @jay [I wrote a tutorial / reference for the equation editor](http://meta.math.stackexchange.com/questions/5020/tex-latex-mathjax-basic-tutorial-and-quick-reference).2012-08-28

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A simple rearrangement shows that $$\tan\theta = \mu\left(1+\frac{M}{m}\right)$$ to obtain numerical values for the equation you'll need the ratio of the masses and the coefficient of friction, in which case you can simply take the arctangent.