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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
0.5/2 = 6.5/x // - 6.5/x
0.5/2-(6.5/x) = 0
0.5/2-6.5*x^-1 = 0
-6.5*x^-1 = -0.25 // : -6.5
x^-1 = 0.03846154
-1 < 0
1/(x^1) = 0.03846154 // * x^1
1 = 0.03846154*x^1 // : 0.03846154
25.99999896 = x^1
x = 25.99999896
x = 25.99999896
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