I've done part (a) but cannot understand the method of how part (b) is done. I've written down the answer I obtained (which is correct) to part (a). Can someone help? Thanks in advance.

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** Q:**
A small smooth ball

of mass

is moving on a horizontal table with speed

when it collides directly with another small smooth ball

of mass

which is at rest on the table. The balls have the same radius and the coefficient of restitution between the balls is

. The direction of motion of

is reversed as a result of the collision.

(a) Find, in terms of

and

. the speeds of

and

immediately after the collision.

In the subsequent motion

strikes a vertical wall, which is perpendicular to the direction of motion of

, and rebounds. The coefficient of restitution between

and the wall is

.

Given that there is a second collision between

and

,

(b) find the range of values of

for which the motion described is possible.

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A:
(a)