In such type of problems, when velocity of one part of a body is given and that of other is required, we first find the relation between the two displacements, then differentiate them with respect to time. Here, if the distance from the corner to the point A is x and that up to B is
y. Then, v=dxdt
and vB=−dxdt (-sign denotes that y is decreasing )
Further, x2+y(2)=t2
Differentiating with respect to time t
2xdxdt+2ydydt=0
xv=yuB
vB=xyv=vcotθ