(yes, differentiating by a partial derivative!), negate it, and then
differentiate the result by x. Then do the same thing by u_y and y,
add the two results together and equate to zero. Mathematically
(yes, differentiating by a partial derivative!), negate it, and then
differentiate the result by x. Then do the same thing by u_y and y,
add the two results together and equate to zero. Mathematically