## Determining radian value tricky sometimes...

Trigonometric ratios and functions, the unit circle, inverse trig functions, identities, trig graphs, etc.
butterflypoo3
Posts: 4
Joined: Tue Apr 14, 2009 12:16 am
Contact:

### Determining radian value tricky sometimes...

For each of the following values of cos theta, determine the radian values of theta if theta is btwn pi and 2pi.

- 1/2 MY answer is theta= pi/3 but textbook says it is 4pi/3....and i have NO idea why...I have this same issue with other questions of this type, I get the RAA right, but not the coefficient

Also,

Terminal arm of an angle in standard position passes thru each of the following points. Find radian value of angle in internal [0, 2pi].

point (-7, 8)
I got sin theta= y/r
= 8/10.63 (using pythagorean theorem)
theta = .752476694 but textbook answer is 2.25....once again, i have no idea how they came to this.

When do i use sin^-1 vs. sin by itself? o.0 HELP!! Unit test tmrw afternoon!!

stapel_eliz
Posts: 1628
Joined: Mon Dec 08, 2008 4:22 pm
Contact:
For each of the following values of cos theta, determine the radian values of theta if theta is btwn pi and 2pi.

- 1/2 MY answer is theta= pi/3...
You might want to review the cosine wave. Is it really negative between $\theta\, =\, 0$ and $\theta\, =\, \frac{\pi}{2}$?
Terminal arm of an angle in standard position passes thru each of the following points. Find radian value of angle in internal [0, 2pi]: point (-7, 8)
You might want to review the x,y-plane (in particular, the Quadrants) and the unit circle. You can up with an angle value in the first quadrant. Is the point (-7, 8) in the first quadrant, or do you need to find a second-quadrant equivalent?
When do i use sin^-1 vs. sin by itself?
When you're taking the sine of an angle, you use the "sine" function. When you are working with a numerical value and wanting to find a corresponding angle value, you use the "inverse sine" function.