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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
5/8 = 40/x // - 40/x
5/8-(40/x) = 0
5/8-40*x^-1 = 0
-40*x^-1 = -5/8 // : -40
x^-1 = 1/64
-1 < 0
1/(x^1) = 1/64 // * x^1
1 = 1/64*x^1 // : 1/64
64 = x^1
x = 64
x = 64
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