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D( x )
x^2 = 0
x^2 = 0
x^2 = 0
1*x^2 = 0 // : 1
x^2 = 0
x = 0
x in (-oo:0) U (0:+oo)
x^2+81/(x^2) = 18 // - 18
x^2+81/(x^2)-18 = 0
x^2+81*x^-2-18 = 0
t_1 = x^2
1*t_1^1+81*t_1^-1-18 = 0
1*t_1^1+81*t_1^-1-18*t_1^0 = 0
(1*t_1^2-18*t_1^1+81*t_1^0)/(t_1^1) = 0 // * t_1^2
t_1^1*(1*t_1^2-18*t_1^1+81*t_1^0) = 0
t_1^1
t_1^2-18*t_1+81 = 0
t_1^2-18*t_1+81 = 0
DELTA = (-18)^2-(1*4*81)
DELTA = 0
t_1 = 18/(1*2)
t_1 = 9 or t_1 = 9
t_1 in { 9, 9}
t_1 = 9
x^2-9 = 0
1*x^2 = 9 // : 1
x^2 = 9
x^2 = 9 // ^ 1/2
abs(x) = 3
x = 3 or x = -3
t_1 = 9
x^2-9 = 0
1*x^2 = 9 // : 1
x^2 = 9
x^2 = 9 // ^ 1/2
abs(x) = 3
x = 3 or x = -3
x in { 3, -3, 3, -3 }
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