Jeanette is playing in a 9-ball pool tournament. She will win if she sinks the 9-ball from the final rack, so she needs to line up her shot precisely. Both the cue ball and the 9-ball have mass m, and the cue ball is hit at an initial speed of vi. Jeanette carefully hits the cue ball into the 9-ball off center, so that when the balls collide, they move away from each other at the same angle θ from the direction in which the cue ball was originally traveling (see figure). Furthermore, after the collision, the cue ball moves away at speed vf, while the 9-ball moves at speed v9. (For the purposes of this problem, assume that the collision is perfectly elastic, neglect friction, and ignore the spinning of the balls.)

Find the angle θ that the 9-ball travels away from the horizontal, as shown in the figure. (I couldn't add the picture unfortunately, but theta is the same for both balls, they spread out with the same angle.)