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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
0.2 = (x-20000)/x // - (x-20000)/x
0.2-((x-20000)/x) = 0
(-1*(x-20000))/x+0.2 = 0
(-1*(x-20000))/x+(0.2*x)/x = 0
0.2*x-1*(x-20000) = 0
20000-0.8*x = 0
(20000-0.8*x)/x = 0
(20000-0.8*x)/x = 0 // * x
20000-0.8*x = 0
20000-0.8*x = 0 // - 20000
-0.8*x = -20000 // : -0.8
x = -20000/(-0.8)
x = 25000
x = 25000
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