# Math Help - Polar Coordinates

1. ## Polar Coordinates

Find the polar coordinates of this point with the given rectangular coordinates. Use 0 < or = theta < 2pi and r > or + to 0.

(-2, -5)

I understand how to get r but how do i get theta expressed in pi?

2. Originally Posted by acherucheril
Find the polar coordinates of this point with the given rectangular coordinates. Use 0 < or = theta < 2pi and r > or + to 0.

(-2, -5)

I understand how to get r but how do i get theta expressed in pi?

Draw a diagram. Construct a right angled triangle in the third quadrant (where (-2, -5) is.
Use trigonometry (tan^-1) to find the acute angle below the neg x-axis ( this must be in radians). Add this angle to pi (to get the total angle anticlockwise from the pos x-axis). Then you'll have theta.

3. Originally Posted by Debsta

Draw a diagram. Construct a right angled triangle in the third quadrant (where (-2, -5) is.
Use trigonometry (tan^-1) to find the acute angle below the neg x-axis ( this must be in radians). Add this angle to pi (to get the total angle anticlockwise from the pos x-axis). Then you'll have theta.
Ok so i do [ tan^-1(-5/-2) + pi ] ?

4. Originally Posted by acherucheril
Ok so i do [ tan^-1(-5/-2) + pi ] ?
Yes, although I'd leave off the neg signs on the 5 and the 2. (It won't make a difference in this case). Find the acute angle then add pi.
Note: If the point was (-2, +5), you'd find the same angle (this time in the second quadrant) and subtract it from pi.
A diagram makes everything clear.

5. Originally Posted by Debsta
Yes, although I'd leave off the neg signs on the 5 and the 2. (It won't make a difference in this case). Find the acute angle then add pi.
Note: If the point was (-2, +5), you'd find the same angle (this time in the second quadrant) and subtract it from pi.
A diagram makes everything clear.
how do you change 248.1985905 into radians?

6. Originally Posted by acherucheril
how do you change 248.1985905 into radians?
Mutiply by pi/180.
Better still switch your calculator to radian mode in the first place

7. so if i changed it to radian mode, it equals 1.37pi. How do i turn that into something thats on the unit circle?

8. nvm again.