soluzione Calcolare ( x y   -   y x ) : [( 1 x2   +   1 y2   -   4 x2 + y2 )·( x + y x2 - xy   +   x - y x2 + xy )]   =     =   ( x y   -   y x ) : [( 1 x2   +   1 y2   -   4 x2 + y2 )·( x + y x(x - y)   +   x - y x(x + y) )]   =     =   x2 - y2 xy : [ y2(x2 + y2) + x2(x2 + y2) - 4x2y2 x2y2(x2 + y2) · (x + y)2 +(x - y)2 x(x - y)(x + y) ]   =     =   x2 - y2 xy : [ x2y2 + y4 + x4 + x2y2 - 4x2y2 x4y2+ x2y4 · x2 + 2xy + y2 + x2 - 2xy + y2 x(x - y)(x + y) ]   =     =   x2 - y2 xy : [ y4 + x4 - 2x2y2 x4y2+ x2y4 · 2x2 + 2y2 x(x - y)(x + y) ]   =     =   x2 - y2 xy : [ (x - y)2(x + y)2 x2y2(x2 + y2) · 2(x2 + y2) x(x - y)(x + y) ]   =     =   x2 - y2 xy : 2(x - y)(x + y) x3y2   =     =   x2 - y2 xy · x3y2 2(x - y)(x + y)   =     =   (x - y)(x + y) xy · x3y2 2(x - y)(x + y)   =     =   x2y 2