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D( z )
z = 0
z = 0
z = 0
z in (-oo:0) U (0:+oo)
z = 60/z // - 60/z
z-(60/z) = 0
z-60*z^-1 = 0
1*z^1-60*z^-1 = 0
(1*z^2-60*z^0)/(z^1) = 0 // * z^2
z^1*(1*z^2-60*z^0) = 0
z^1
z^2-60 = 0
z^2-60 = 0
DELTA = 0^2-(-60*1*4)
DELTA = 240
DELTA > 0
z = (240^(1/2)+0)/(1*2) or z = (0-240^(1/2))/(1*2)
z = 2*15^(1/2) or z = -2*15^(1/2)
z in { -2*15^(1/2), 2*15^(1/2)}
z in { -2*15^(1/2), 2*15^(1/2) }
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