How to find the angle{theta} when the question is given like this

que: two equal discs initially at rest are in contact on a table. a third disc of same mass but of double radius strikes them symmetrically and itself comes to rest after impact.the coeficient of restitution is

1] 2/3

2] 9/16

3] 3/4

4] 8/17

let speed before collision be u, and after collision be v. conservation of linear momentum in the direction of motion of the bigger disk gives 2mv(2√2)/3=mu, v=3u/4√2, let 'theta' be the angle between u and v. coefficient of restitution

=v/(u(cosθ))=(3u/4√2)/(u(√8)/3)=9/16

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