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mill_drill@yahoogroups.com, "bob_kellock" <bob@...> wrote:

> Convert the Cartesian coordinates to polar, add the angular

> displacement to the polar angle and convert back to Cartesian.

>

> As an example say x1=3, y1=8 and the displacement is 120 deg.

> Initial polar coordinates:

> Angle A1 = arctan(3/8) = 20.556

> Radius R = Square root(3 squared + 8 squared) = 8.544

>

> A2 = A1 + displacement = 140.556

> x2 = R*sin(140.556) = -6.598

> y2 = R*cos(140.556) = 5.428

I suggest avoiding this method. Simple example: let x1=-3 and y1=-

8. Plug that in and you'll get the incorrect result. Inverse trig

function are very dangerous to use, since they usually have multiple

possible results. I tell my students this all of the time and still

get programs sometimes that fail for certain angles because they used

inverse trig function. The previous solution using sin and cos is a

much better choice. If you have a calculator that can do rectangular

to polar conversions for you, that can work very well, since they use

a better way to do arctan of write a C program using the atan2

function.

Charles