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In my Trig class we have begun working on graphing the trig functions and working with radians and I'm trying to wrap my head around them.

At the moment I'm having trouble understanding radian measures and how to find where certain points lie on the unit circle and how to know what quadrant they are in.

For example, we are to find the reference angle of $\frac{5\pi}{6}$. My book says it terminates in QII.

This may be a dumb question, but how does one figure this out? What am I missing? How do you know that $\frac{\pi}{2} < \frac{5\pi}{6} < \pi$?

Thanks in advance...

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    I probably should have said "from the portion $x$-axis that lies adjacent to the quadrant the angle 'sits' in." The intuitive way of looking at it. E.g. the reference angle of $\frac{5\pi}{3}$ is $2\pi - \frac{5\pi}{3} = \frac{\pi}{3}$.2011-03-06

3 Answers 3

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Think of the fractions without π: $\frac{1}{2}<\frac{5}{6}<1$ (or, alternately, since we're dealing with sixths, 5 sixths is between 3 sixths ($\frac{1}{2}$) and 6 sixths ($1$).

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Write whatever the angle is in radians as $\frac{p}{q}\pi$. Since going around the top half of the circle once is equivalent to $pi$ radians, divide the top half up into $q$ pieces, and the bottom half into $q$ pieces (like pie slices). So you're dividing your whole circle into $2q$ pieces. Now, starting with the top of the first pie slice above the $x$-axis on the right side being 1, count out $p$ pie slices. Where you end is $p$ "copies" of $\frac{\pi}{q}$ and is the quadrant you belong in.

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We know that $\pi = 6 \pi/6$ and $\pi/2 = 3 \pi/6$.