# CAST diagram help

can somebody explain how I would use the cast diagram to find the solutions for cos^-1(-2/3) in the interval 0 and 360. I've learnt what to do if the calculator answer is positive or negative, but this seems to be a bit different. I'm not sure how im supposed to plot 131.8 degrees in the cos and all quadrant on the diagram
(edited 5 months ago)
Original post by Y12EC
can somebody explain how I would use the cast diagram to find the solutions for cos^-1(-2/3) in the interval 0 and 360. I've learnt what to do if the calculator answer is positive or negative, but this seems to be a bit different. I'm not sure how im supposed to plot 131.8 degrees in the cos and all quadrant on the diagram

Its basically a reflection in the xaxis so 360-131.8 as in
https://www.desmos.com/calculator/trv6uxvhkg
The x=-2/3 corresponds to a vertical line and where it intersects with the unit circle gives the two solutions for (arc)cos. sin is similar except it corresponds to a horizonntal line.
(edited 5 months ago)