unity - how to set angle between points of a bezier curve

From the bezier curve image above, how can I set the angle r manually, so that changing it does not affect the distance between A and B?

Answer

// Form the direction the tangent should point when the angle is zero.
var zeroAngleDirection = (D - A).normalized;

// Compute a rotation by the desired angle, clockwise about the positive z axis.
var twist = Quaternion.Euler(0, 0, -angle);

// Set point B a desired distance away from A, in the rotated direction.
B = A + twist * zeroAngleDirection * desiredLength;