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D( x )
8*x = 0
8*x = 0
8*x = 0
8*x = 0 // : 8
x = 0
x in (-oo:0) U (0:+oo)
(12*30000)/(8*x) = 9/4 // - 9/4
(12*30000)/(8*x)-(9/4) = 0
(12*30000)/(8*x)-9/4 = 0
45000*x^-1 = 9/4 // : 45000
x^-1 = 1/20000
-1 < 0
1/(x^1) = 1/20000 // * x^1
1 = 1/20000*x^1 // : 1/20000
20000 = x^1
x = 20000
x = 20000
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