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D( x )
(x+4)/(49-x^2) = 0
49-x^2 = 0
3*x+21 = 0
(x+4)/(49-x^2) = 0
(x+4)/(49-x^2) = 0
x+4 = 0 // - 4
x = -4
49-x^2 = 0
49-x^2 = 0
-1*x^2 = -49 // : -1
x^2 = 49
x^2 = 49 // ^ 1/2
abs(x) = 7
x = 7 or x = -7
3*x+21 = 0
3*x+21 = 0
3*x+21 = 0 // - 21
3*x = -21 // : 3
x = -21/3
x = -7
x in (-oo:-7) U (-7:-4) U (-4:7) U (7:+oo)
(x/(3*x+21))/((x+4)/(49-x^2)) = 0
(x*(49-x^2))/((3*x+21)*(x+4)) = 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
( x )
x = 0
x in { 7}
x in { -7}
x = 0
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