mathematics - Tangent to a circle through a point

I'm trying to figure out how to calculate the following angle:

I know center (p1) and radius (r) of a circle. Given a point p3 I want to calculate the angle a such as the tangent (tan) of the circle at point p4, points in p3 direction.

Here's a figure:

Doesn anyone know how to do that?

Answer

Let b be the angle between vectors p1p2 and p1p3. Its value can be computed as:

b = pi - atan2(p1p3.y, p1p3.x)

The angle between p1p4 and p1p3 is b-a. Since p1p3p4 is a right-angled triangle, we know that cos(b-a) is the distance p1p4 divided by the distance p1p3.

The answer is then:

a = pi - atan2(p1p3.y, p1p3.x) - acos(r / length(p1p3))

Replacing the second - with + will give you the second possible solution.