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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
0.25 = (x-20)/x // - (x-20)/x
0.25-((x-20)/x) = 0
(-1*(x-20))/x+0.25 = 0
(-1*(x-20))/x+(0.25*x)/x = 0
0.25*x-1*(x-20) = 0
20-0.75*x = 0
(20-0.75*x)/x = 0
(20-0.75*x)/x = 0 // * x
20-0.75*x = 0
20-0.75*x = 0 // - 20
-0.75*x = -20 // : -0.75
x = -20/(-0.75)
x = 26.66666667
x = 26.66666667
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