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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
13/21 = 299/x // - 299/x
13/21-(299/x) = 0
13/21-299*x^-1 = 0
-299*x^-1 = -13/21 // : -299
x^-1 = 1/483
-1 < 0
1/(x^1) = 1/483 // * x^1
1 = 1/483*x^1 // : 1/483
483 = x^1
x = 483
x = 483
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