soluzione Calcolare [1 + 2( 1 x + y - 1 y ) - y( 1 x + y + x y2 )] · ( 1 xy - 2y + 1 xy + 2y + 2 x2 - 4 )   =     =   [1 + 2( 1 x + y - 1 y ) - y( 1 x + y + x y2 )] · ( 1 y(x - 2) + 1 y(x + 2) + 2 (x - 2)(x + 2) )   =     =   [1 + 2· y - (x + y) y(x + y) - y· y2 +x(x + y) y2(x + y) ] · x + 2 + x - 2 + 2y y(x - 2)(x + 2)   =     =   [1 + 2· y - x - y y(x + y) - y· y2 +x2 + xy y2(x + y) ] · x + 2 + x - 2 + 2y y(x - 2)(x + 2)   =     =   [1 + 2· - x y(x + y) - y· y2 +x2 + xy y2(x + y) ] · 2x + 2y y(x - 2)(x + 2)   =     =   [1 - 2x y(x + y) - y2 +x2 + xy y(x + y) ] · 2x + 2y y(x - 2)(x + 2)   =     =   y(x + y) - 2x -(y2 + x2 + xy) y(x + y) · 2x + 2y y(x - 2)(x + 2)   =     =   xy + y2 - 2x -y2 - x2 - xy) y(x + y) · 2x + 2y y(x - 2)(x + 2)   =     =   - x2 - 2x y(x + y) · 2x + 2y y(x - 2)(x + 2)   =     =   - x(x - 2) y(x + y) · 2(x + y) y(x - 2)(x + 2)   =     =   2x -   y2(x + 2))