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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
20/2 = 60/x // - 60/x
20/2-(60/x) = 0
20/2-60*x^-1 = 0
-60*x^-1 = -10 // : -60
x^-1 = 1/6
-1 < 0
1/(x^1) = 1/6 // * x^1
1 = 1/6*x^1 // : 1/6
6 = x^1
x = 6
x = 6
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