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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
21*x-40 = 21/x // - 21/x
21*x-(21/x)-40 = 0
21*x-21*x^-1-40 = 0
21*x^1-21*x^-1-40*x^0 = 0
(21*x^2-40*x^1-21*x^0)/(x^1) = 0 // * x^2
x^1*(21*x^2-40*x^1-21*x^0) = 0
x^1
21*x^2-40*x-21 = 0
21*x^2-40*x-21 = 0
DELTA = (-40)^2-(-21*4*21)
DELTA = 3364
DELTA > 0
x = (3364^(1/2)+40)/(2*21) or x = (40-3364^(1/2))/(2*21)
x = 7/3 or x = -3/7
x in { -3/7, 7/3}
x in { -3/7, 7/3 }
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