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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
(2*x^-2)/3-((7*x^-1)/3)-20 = 0
(-7/3)*x^-1+(2*x^-2)/3-20 = 0
2/3*x^-2-7/3*x^-1-20 = 0
t_1 = x^-1
2/3*t_1^2-7/3*t_1^1-20 = 0
2/3*t_1^2-7/3*t_1-20 = 0
DELTA = (-7/3)^2-(-20*2/3*4)
DELTA = 529/9
DELTA > 0
t_1 = ((529/9)^(1/2)+7/3)/(2/3*2) or t_1 = (7/3-(529/9)^(1/2))/(2/3*2)
t_1 = 15/2 or t_1 = -4
t_1 = -4
x^-1+4 = 0
1*x^-1 = -4 // : 1
x^-1 = -4
-1 < 0
1/(x^1) = -4 // * x^1
1 = -4*x^1 // : -4
-1/4 = x^1
x = -1/4
t_1 = 15/2
x^-1-15/2 = 0
1*x^-1 = 15/2 // : 1
x^-1 = 15/2
-1 < 0
1/(x^1) = 15/2 // * x^1
1 = 15/2*x^1 // : 15/2
2/15 = x^1
x = 2/15
x in { -1/4, 2/15 }
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