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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
80/x = 2*y-z // - 2*y-z
80/x-(2*y)+z = 0
80/x-2*y+z = 0
80*x^-1 = -(z-2*y) // : 80
x^-1 = (-(z-2*y))/80
-1 < 0
1/(x^1) = (-(z-2*y))/80 // * x^1
1 = x^1*((-(z-2*y))/80) // : (-(z-2*y))/80
-80*(z-2*y)^-1 = x^1
x = -80*(z-2*y)^-1
x = -80*(z-2*y)^-1
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