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D( x )
x-13 = 0
x-13 = 0
x-13 = 0
x-13 = 0 // + 13
x = 13
x in (-oo:13) U (13:+oo)
99/(x-13) = 11 // - 11
99/(x-13)-11 = 0
99/(x-13)+(-11*(x-13))/(x-13) = 0
99-11*(x-13) = 0
242-11*x = 0
(242-11*x)/(x-13) = 0
(242-11*x)/(x-13) = 0 // * x-13
242-11*x = 0
242-11*x = 0 // - 242
-11*x = -242 // : -11
x = -242/(-11)
x = 22
x = 22
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