докажите что x+(y+(z+(t+u))) =x+y+z+t+u

x+(y+(z+(t+u))) =x+y+z+t+u

x+(y+(z+(t+u))) = x+(y+(z+t+u)) = x+(y+z+t+u) = x+y+z+t+u

x+y+z+t+u = x+y+z+t+u