Find the speed of the box-3, if box-1 and box-2 are moving with speeds `v_(1)` and `v_(2)` as shown in figure when the string makes an angle `theta_(1)` and `theta_(2)` with the horizontal at its left and right end.

Answer

`[((v_(1) + v_(2))cos theta_(1))/(cos theta_(2))-v_(2)]`