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F(x,y) = (1/π^2) [π/2 + arctan(x/2)].[π/2 + arctan(y/2)]
∂F/∂y =(1/π^2) [π/2 + arctan(x/2)]. [ (1/2)/( 1+ (y/2)^2) ]
=(1/π^2) [π/2 + arctan(x/2)]. [ 2/( 4+ y^2) ]
∂^2F/∂x∂y
=∂/∂x (∂F/∂y)
=(1/π^2) [ 2/( 4+ y^2) ] [ 2/(4+x^2) ]
= (4/π^2) { 1/[( 4+ x^2) .(4+x^2) ] }
∂F/∂y =(1/π^2) [π/2 + arctan(x/2)]. [ (1/2)/( 1+ (y/2)^2) ]
=(1/π^2) [π/2 + arctan(x/2)]. [ 2/( 4+ y^2) ]
∂^2F/∂x∂y
=∂/∂x (∂F/∂y)
=(1/π^2) [ 2/( 4+ y^2) ] [ 2/(4+x^2) ]
= (4/π^2) { 1/[( 4+ x^2) .(4+x^2) ] }
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