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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
80/x = 8.21 // - 8.21
80/x-8.21 = 0
80*x^-1 = 8.21 // : 80
x^-1 = 0.102625
-1 < 0
1/(x^1) = 0.102625 // * x^1
1 = 0.102625*x^1 // : 0.102625
9.74421437 = x^1
x = 9.74421437
x = 9.74421437
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