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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
4 = x-(21/x) // - x-(21/x)
21/x-x+4 = 0
21*x^-1-1*x^1+4*x^0 = 0
(4*x^1-1*x^2+21*x^0)/(x^1) = 0 // * x^2
x^1*(4*x^1-1*x^2+21*x^0) = 0
x^1
4*x-x^2+21 = 0
4*x-x^2+21 = 0
DELTA = 4^2-(-1*4*21)
DELTA = 100
DELTA > 0
x = (100^(1/2)-4)/(-1*2) or x = (-100^(1/2)-4)/(-1*2)
x = -3 or x = 7
x in { -3, 7}
x in { -3, 7 }
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