Aditya Agarwal but, sir that is what i have done.
due to movement of 2 a dist. x to the right, at that instant,
the length of the string will increase by 2.x.(csc(theta))
and because of movement of 1, it will decrease by y
now differentiating,
-v1 + 2.(v2).csc(theta) = 0.
Akash Anand By your approach, x and y both are changing, and the statement that you are making that movement of 2x will increease the length by 2x cosec theta is wrong. Think about it.