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D( u )
3*u^3 = 0
3*u^3 = 0
3*u^3 = 0
3*u^3 = 0 // : 3
u^3 = 0
u = 0
u in (-oo:0) U (0:+oo)
(12*u^6-(15*u^4))/(3*u^3) = 0
(12*u^6-15*u^4)/(3*u^3) = 0
12*u^6-15*u^4 = 0
3*u^4*(4*u^2-5) = 0
4*u^2 = 5 // : 4
u^2 = 5/4
u^2 = 5/4 // ^ 1/2
abs(u) = (5/4)^(1/2)
u = (5/4)^(1/2) or u = -(5/4)^(1/2)
3*u^4*(u-(5/4)^(1/2))*(u+(5/4)^(1/2)) = 0
(3*u^4*(u-(5/4)^(1/2))*(u+(5/4)^(1/2)))/(3*u^3) = 0
( 3*u^4 )
3*u^4 = 0 // : 3
u^4 = 0
u = 0
( u+(5/4)^(1/2) )
u+(5/4)^(1/2) = 0 // - (5/4)^(1/2)
u = -(5/4)^(1/2)
( u-(5/4)^(1/2) )
u-(5/4)^(1/2) = 0 // + (5/4)^(1/2)
u = (5/4)^(1/2)
u in { 0}
u in { -(5/4)^(1/2), (5/4)^(1/2) }
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