code: public void setArcByTangent (XY p1, XY p2, XY p3, float radius) {
          // use simple geometric calculations of arc center, radius and angles by tangents
          float a1 = -FloatMath.atan2(p1.y() - p2.y(), p1.x() - p2.x());
          float a2 = -FloatMath.atan2(p3.y() - p2.y(), p3.x() - p2.x());
          float am = (a1 + a2) / 2f;
          float ah = a1 - am;
          float d = radius / Math.abs(FloatMath.sin(ah));
          float x = p2.x() + d * FloatMath.cos(am);
          float y = p2.y() - d * FloatMath.sin(am);
          ah = ah >= 0f ? FloatMath.PI * 1.5f - ah : FloatMath.PI * 0.5f - ah;
          a1 = normAngle(FloatMath.toDegrees(am - ah));
          a2 = normAngle(FloatMath.toDegrees(am + ah));
          float delta = a2 - a1;
          if (delta <= 0f) {
              delta += 360f;
          }
          setArcByCenter(x, y, radius, a1, delta, type);
      }
docs: Sets the location, size, angular extents, and closure type of this arc based on the
  specified values.