Question About Unit Circle (CircularFunction) of a Trig Func

1. Oct 20, 2015

basty

Please take a look below example (the attached image below).

How do I know that the angle $\sin (\frac{7π}{4})$ is corresponds to the coordinates $(\frac{\sqrt {2}}{2}, -\frac{\sqrt{2}}{2})$?

I know that $\frac{7π}{4}$ is 315°.

2. Oct 20, 2015

BvU

Did you draw a unit circle and mark e.g. the $7\pi \over 4$ angle ?

3. Oct 20, 2015

basty

Should I?

4. Oct 20, 2015

BvU

Yes

5. Oct 20, 2015

Ssnow

subtract $2\pi$ that is $\frac{7}{4}\pi -2\pi=\frac{7-8}{4}\pi=-\frac{\pi}{4}$...

6. Oct 22, 2015

HallsofIvy

Staff Emeritus
If you draw a line from (0, 0) with length 1 and making angle $\theta$ with the x-angle and drop a perpendicular to the x-axis, then the distance to the foot of that perpendicular, along the x-axis is the "near side" of a right triangle with angle $\theta$ and hypotenuse 1. Similarly, the length of the perpendicular, parallel to the y-axis, is the "opposite side".

7. Oct 22, 2015

aikismos

Well, I think any high school teacher I knew when I was teaching would have a simple answer:

Memorize the unit circle (which isn't so hard to do if notice the angular symmetries and remember the mnemonic device All Students Take Calculus in order to remember the signs).