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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
(13/3)*x = 4/x-(1/21) // - 4/x-(1/21)
(13/3)*x-(4/x)+1/21 = 0
(13/3)*x-4*x^-1+1/21 = 0
13/3*x^1-4*x^-1+1/21*x^0 = 0
(13/3*x^2+1/21*x^1-4*x^0)/(x^1) = 0 // * x^2
x^1*(13/3*x^2+1/21*x^1-4*x^0) = 0
x^1
(13/3)*x^2+(1/21)*x-4 = 0
(13/3)*x^2+(1/21)*x-4 = 0
DELTA = (1/21)^2-(-4*4*(13/3))
DELTA = 30577/441
DELTA > 0
x = ((30577/441)^(1/2)-(1/21))/(2*(13/3)) or x = (-(1/21)-(30577/441)^(1/2))/(2*(13/3))
x = 3/26*((30577/441)^(1/2)-1/21) or x = -3/26*((30577/441)^(1/2)+1/21)
x in { -3/26*((30577/441)^(1/2)+1/21), 3/26*((30577/441)^(1/2)-1/21)}
x in { -3/26*((30577/441)^(1/2)+1/21), 3/26*((30577/441)^(1/2)-1/21) }
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