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D( x )
(28-(4*x))/(7*x) = 0
7*x = 0
5*x^3 = 0
(28-(4*x))/(7*x) = 0
(28-(4*x))/(7*x) = 0
(28-4*x)/(7*x) = 0
28-4*x = 0 // - 28
-4*x = -28 // : -4
x = -28/(-4)
x = 7
7*x = 0
7*x = 0
7*x = 0 // : 7
x = 0
5*x^3 = 0
5*x^3 = 0
5*x^3 = 0 // : 5
x^3 = 0
x = 0
x in (-oo:0) U (0:7) U (7:+oo)
((49-x^2)/(5*x^3))/((28-(4*x))/(7*x)) = 0
((49-x^2)/(5*x^3))/((28-4*x)/(7*x)) = 0
(7*x*(49-x^2))/(5*x^3*(28-4*x)) = 0
( 49-x^2 )
-1*x^2 = -49 // : -1
x^2 = 49
x^2 = 49 // ^ 1/2
abs(x) = 7
x = 7 or x = -7
( 7*x )
7*x = 0 // : 7
x = 0
x in { 7}
x in { 0}
x = -7
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