given the set of coordinates (-1035,-410) how would i go about finding another set of coordinates that is 120 degrees away?

- Apr 19th 2012, 10:06 AMjustinjrichardsGiven one set of coordinates, find another set that is 120 degrees away.
given the set of coordinates (-1035,-410) how would i go about finding another set of coordinates that is 120 degrees away?

- Apr 19th 2012, 10:14 AMPlatoRe: Given one set of coordinates, find another set that is 120 degrees away.
- Apr 19th 2012, 12:34 PMjustinjrichardsRe: Given one set of coordinates, find another set that is 120 degrees away.
its been a while since i took trig. so my terminology may be incorrect, but if the given coordinates are on the circumference of the circle then there could be two more sets of coordinates on the circle that are 120 degrees apart. one set clockwise 120 degrees and another set counter clockwise 120 degrees. the center of the circle would be at (0,0) i believe.

- Apr 19th 2012, 12:59 PMPlatoRe: Given one set of coordinates, find another set that is 120 degrees away.
If $\displaystyle (-1035,-410)$ is on a circle centered at $\displaystyle (0,0)$ then the radius is $\displaystyle 1113.25$.

You posted this in a trigonometry forum. The answer I will give may not look correct in trig terms.

This is done with a rotation matrix, a transformation.

$\displaystyle \left( {x\cos (\theta ) - y\sin (\theta ),x\sin (\theta ) + y\cos (\theta )} \right)$ is the resulting point of rotating the point $\displaystyle (x,y)$ about $\displaystyle (0,0)$ through an angle of $\displaystyle \theta$. - Apr 19th 2012, 03:25 PMskeeterRe: Given one set of coordinates, find another set that is 120 degrees away.
let $\displaystyle (x_1,y_1)$ be the starting set of coordinates.

$\displaystyle r = \sqrt{x_1^2 + y_1^2}$

since $\displaystyle x_1$ and $\displaystyle y_1$ are both negative, the angle of the ray that passes thru $\displaystyle (x_1,y_1)$ is $\displaystyle \theta = \arctan\left(\frac{y_1}{x_1}\right) + 180^\circ$

$\displaystyle 120^\circ$ "away" could be $\displaystyle (\theta + 120^\circ)$ or $\displaystyle (\theta - 120^\circ)$

new coordinates could be $\displaystyle (r\cos(\theta + 120^\circ),r\sin(\theta + 120^\circ))$ or $\displaystyle (r\cos(\theta - 120^\circ),r\sin(\theta - 120^\circ))$ - Apr 19th 2012, 03:47 PMPlatoRe: Given one set of coordinates, find another set that is 120 degrees away.
With regard to reply #5 there is a way to simply cut to the chase directly.

Using $\displaystyle \left( {x\cos (\theta ) - y\sin (\theta ),x\sin (\theta ) + y\cos (\theta )} \right)$ let $\displaystyle x=-1035~\&~y=-410$ then take two cases $\displaystyle \theta =\pm 120^o$.