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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
60*(80/x) = 20 // - 20
60*(80/x)-20 = 0
4800*x^-1 = 20 // : 4800
x^-1 = 1/240
-1 < 0
1/(x^1) = 1/240 // * x^1
1 = 1/240*x^1 // : 1/240
240 = x^1
x = 240
x = 240
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