the interval is x and y between 0.25pi and 0.5pi (the square thing kind of confused me!)

I got

∂u/∂x = siny sin(2x+y) =0

siny = 0

y = 0 ± npi (not in interval so doesn't count ; npi means integer number of pi)

OR

2x +y =0

Also,

∂u/∂y = sinx sin(2y+x) =0

Again sinx =0 ; x = 0 ±npi is outside the given interval

2y + x =0

if you try to find x:

2x -0.5x = 0 ±npi

x = n * 2/3 * pi

even with n = 1 this is outside the interval!!

Please help!?