\(F_x=-\frac{\partial U}{\partial x}=\sin (x+y)\)
\(F _{ y }=-\frac{\partial U }{\partial x }=\sin ( x + y )\)
\(\left.F _{ x }=\sin ( x + y )\right]_{(0, \pi / 4)}=\frac{1}{\sqrt{2}}\),
\(\left.F _{ y }=\sin ( x + y )\right]_{(0, \pi / 4)}=\frac{1}{\sqrt{2}} \quad \therefore F =\frac{1}{\sqrt{2}}[\hat{ i }+\hat{ j }]\)