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D( x )
x = 0
x^2 = 0
x = 0
x = 0
x^2 = 0
x^2 = 0
1*x^2 = 0 // : 1
x^2 = 0
x = 0
x in (-oo:0) U (0:+oo)
1-(4/x) = 21/(x^2) // - 21/(x^2)
1-(4/x)-(21/(x^2)) = 0
1-4*x^-1-21*x^-2 = 0
t_1 = x^-1
1-21*t_1^2-4*t_1^1 = 0
1-21*t_1^2-4*t_1 = 0
DELTA = (-4)^2-(-21*1*4)
DELTA = 100
DELTA > 0
t_1 = (100^(1/2)+4)/(-21*2) or t_1 = (4-100^(1/2))/(-21*2)
t_1 = -1/3 or t_1 = 1/7
t_1 = -1/3
x^-1+1/3 = 0
1*x^-1 = -1/3 // : 1
x^-1 = -1/3
-1 < 0
1/(x^1) = -1/3 // * x^1
1 = -1/3*x^1 // : -1/3
-3 = x^1
x = -3
t_1 = 1/7
x^-1-1/7 = 0
1*x^-1 = 1/7 // : 1
x^-1 = 1/7
-1 < 0
1/(x^1) = 1/7 // * x^1
1 = 1/7*x^1 // : 1/7
7 = x^1
x = 7
x in { -3, 7 }
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