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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
-13/x = -40 // + 40
40-13/x = 0
-13*x^-1 = -40 // : -13
x^-1 = 40/13
-1 < 0
1/(x^1) = 40/13 // * x^1
1 = 40/13*x^1 // : 40/13
13/40 = x^1
x = 13/40
x = 13/40
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