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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
90/x-(90/x)+6 = 5/x // - 5/x
90/x-(90/x)-(5/x)+6 = 0
90/x-90*x^-1-5*x^-1+6 = 0
-5*x^-1 = -6 // : -5
x^-1 = 6/5
-1 < 0
1/(x^1) = 6/5 // * x^1
1 = 6/5*x^1 // : 6/5
5/6 = x^1
x = 5/6
x = 5/6
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