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u is constant along curves in the direction (5,2) because the directional derivative of u in that direction is zero. If the rate of change of a function is zero in a specific direction, the function's value does not change as one moves along any curve tangent to that direction.

Conditions: (5,2)·∇$u=0$.; The curves considered are tangent to the direction (5,2).

u is constant along curves in the direction (5,2) because the directional derivative of u in that direction is zero. If the rate of change of a function is zero in a specific direction, the function's value does not change as one moves along any curve tangent to that direction.

Conditions: (5,2)·∇$u=0$.; The curves considered are tangent to the direction (5,2).