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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
10/x = 65/40 // - 65/40
10/x-(65/40) = 0
10/x-13/8 = 0
10*x^-1 = 13/8 // : 10
x^-1 = 13/80
-1 < 0
1/(x^1) = 13/80 // * x^1
1 = 13/80*x^1 // : 13/80
80/13 = x^1
x = 80/13
x = 80/13
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