The potential energy of a force field is given by  U=sin(x+y).The force acting on the particle at position (π4,π4)  is

F→=−∂U∂xi^−∂U∂yj^−∂U∂zk^ F→=−cos(x+y)i^−cos(x+y)j^F→=−cos(x+y)  (i^+j^) At (π4,π4)F→=−cos(π4+π4)(i→+j^) F→=0→        F=0