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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
(80-x)/x = 0.33 // - 0.33
(80-x)/x-0.33 = 0
(80-x)/x+(-0.33*x)/x = 0
80-x-0.33*x = 0
80-1.33*x = 0
(80-1.33*x)/x = 0
(80-1.33*x)/x = 0 // * x
80-1.33*x = 0
80-1.33*x = 0 // - 80
-1.33*x = -80 // : -1.33
x = -80/(-1.33)
x = 60.15037594
x = 60.15037594
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