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D( x )
7*x-21 = 0
7*x-21 = 0
7*x-21 = 0
7*x-21 = 0 // + 21
7*x = 21 // : 7
x = 21/7
x = 3
x in (-oo:3) U (3:+oo)
(3*x^3-(9*x))/(7*x-21) = 0
(3*x^3-9*x)/(7*x-21) = 0
3*x^3-9*x = 0
3*x*(x^2-3) = 0
1*x^2 = 3 // : 1
x^2 = 3
x^2 = 3 // ^ 1/2
abs(x) = 3^(1/2)
x = 3^(1/2) or x = -3^(1/2)
3*x*(x-3^(1/2))*(x+3^(1/2)) = 0
(3*x*(x-3^(1/2))*(x+3^(1/2)))/(7*x-21) = 0
( 3*x )
3*x = 0 // : 3
x = 0
( x+3^(1/2) )
x+3^(1/2) = 0 // - 3^(1/2)
x = -3^(1/2)
( x-3^(1/2) )
x-3^(1/2) = 0 // + 3^(1/2)
x = 3^(1/2)
x in { 0, -3^(1/2), 3^(1/2) }
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