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