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D( x )
-4*u^5*x^4 = 0
-4*u^5*x^4 = 0
-4*u^5*x^4 = 0
-4*u^5*x^4 = 0 // : -4*u^5
x^4 = 0
x = 0
x in (-oo:0) U (0:+oo)
(28*u^7*x^2-15*u^7*x^5)/(-4*u^5*x^4) = 0
28*u^7*x^2-15*u^7*x^5 = 0
u^7*x^2*(28-15*x^3) = 0
-15*x^3 = -28 // : -15
x^3 = 28/15
x^3 = 28/15 // ^ 1/3
x = (28/15)^(1/3)
u^7*x^2*(x-(28/15)^(1/3)) = 0
(u^7*x^2*(x-(28/15)^(1/3)))/(-4*u^5*x^4) = 0
( u^7*x^2 )
u^7*x^2 = 0 // : u^7
x^2 = 0
x = 0
( x-(28/15)^(1/3) )
x-(28/15)^(1/3) = 0 // + (28/15)^(1/3)
x = (28/15)^(1/3)
x in { 0}
x = (28/15)^(1/3)
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