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D( x )
x = 0
x-4 = 0
x = 0
x = 0
x-4 = 0
x-4 = 0
x-4 = 0 // + 4
x = 4
x in (-oo:0) U (0:4) U (4:+oo)
5/x = 1/(x-4) // - 1/(x-4)
5/x-(1/(x-4)) = 0
5/x-(x-4)^-1 = 0
5/x-1/(x-4) = 0
(-1*x)/(x*(x-4))+(5*(x-4))/(x*(x-4)) = 0
5*(x-4)-1*x = 0
4*x-20 = 0
(4*x-20)/(x*(x-4)) = 0
(4*x-20)/(x*(x-4)) = 0 // * x*(x-4)
4*x-20 = 0
4*x-20 = 0 // + 20
4*x = 20 // : 4
x = 20/4
x = 5
x = 5
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