# Given one endpt on an arc of a circle and the radius and arc angle, how to calculate the other endpt of the arc?

I have a circle with an arc beginning at point (x,y). The radius is r, the arc angle(w/ respect to center) is theta. How do I calculate the end point of the arc (a,b) ?

I know that the arclength=radius*(arc angle)

I can't seem to find an easy way to solve this, I think the way to go is with parametric equations but I'm not sure.

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With the given information, all you can say is that $(a,b)$ is at a distance of $2r\sin(\theta/2)$ from $(x,y)$. What you need to pin the point down is some information about direction, for example: the slope of the tangent to the arc at $(x,y)$, or the slope of the line segment joining $(x,y)$ and $(a,b)$, or the angle with respect to the $x$-axis instead of the slope of either; that sort of thing. Do you see why? – Rahul Narain Feb 6 at 5:41

$\alpha = \arctan \left ( \frac{p1.y-cp.y}{p1.x-cp.x} \right )$
$p2.x = cp.x + r * cos (\alpha +\theta )$
$p2.y = cp.y + r * sin (\alpha +\theta )$