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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
300/x = 8/320 // - 8/320
300/x-(8/320) = 0
300/x-1/40 = 0
300*x^-1 = 1/40 // : 300
x^-1 = 1/12000
-1 < 0
1/(x^1) = 1/12000 // * x^1
1 = 1/12000*x^1 // : 1/12000
12000 = x^1
x = 12000
x = 12000
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