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D( x )
3*x^3 = 0
3*x^3 = 0
3*x^3 = 0
3*x^3 = 0 // : 3
x^3 = 0
x = 0
x in (-oo:0) U (0:+oo)
(21*x^6-(24*x^4))/(3*x^3) = 0
(21*x^6-24*x^4)/(3*x^3) = 0
21*x^6-24*x^4 = 0
3*x^4*(7*x^2-8) = 0
7*x^2 = 8 // : 7
x^2 = 8/7
x^2 = 8/7 // ^ 1/2
abs(x) = (8/7)^(1/2)
x = (8/7)^(1/2) or x = -(8/7)^(1/2)
3*x^4*(x-(8/7)^(1/2))*(x+(8/7)^(1/2)) = 0
(3*x^4*(x-(8/7)^(1/2))*(x+(8/7)^(1/2)))/(3*x^3) = 0
( 3*x^4 )
3*x^4 = 0 // : 3
x^4 = 0
x = 0
( x+(8/7)^(1/2) )
x+(8/7)^(1/2) = 0 // - (8/7)^(1/2)
x = -(8/7)^(1/2)
( x-(8/7)^(1/2) )
x-(8/7)^(1/2) = 0 // + (8/7)^(1/2)
x = (8/7)^(1/2)
x in { 0}
x in { -(8/7)^(1/2), (8/7)^(1/2) }
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