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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
900/x = 80 // - 80
900/x-80 = 0
900*x^-1 = 80 // : 900
x^-1 = 4/45
-1 < 0
1/(x^1) = 4/45 // * x^1
1 = 4/45*x^1 // : 4/45
45/4 = x^1
x = 45/4
x = 45/4
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