Question: Problems in Solving a complex Equation

Hello, im a current user of Maple 16, i have a problem in finding a minimum of equation ( equation of a external work equating the internal energy dissipation) f= E-W ( im studying a slope stability with limit analysis)

the equation is:

i need to find x, y: angles of the slope and must be between [0..pi/2] to have a logic solution.

So, to find the minimum i must solve (df/dx=0 and df/dy=0).



i used the command (fsolve):

but i found x=6.23 and y=-9.10 and its not logic to have those reults because i need to find 2 angles between [0, pi/2].

Please can you help me in this, because i couldn't find a solution for this problemm.

Thank you

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