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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
80/100 = 20/x // - 20/x
80/100-(20/x) = 0
80/100-20*x^-1 = 0
-20*x^-1 = -4/5 // : -20
x^-1 = 1/25
-1 < 0
1/(x^1) = 1/25 // * x^1
1 = 1/25*x^1 // : 1/25
25 = x^1
x = 25
x = 25
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