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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
6/80 = 15/x // - 15/x
6/80-(15/x) = 0
6/80-15*x^-1 = 0
-15*x^-1 = -3/40 // : -15
x^-1 = 1/200
-1 < 0
1/(x^1) = 1/200 // * x^1
1 = 1/200*x^1 // : 1/200
200 = x^1
x = 200
x = 200
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