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D( x )
25-x^2 = 0
25-x^2 = 0
25-x^2 = 0
-1*x^2 = -25 // : -1
x^2 = 25
x^2 = 25 // ^ 1/2
abs(x) = 5
x = 5 or x = -5
x in (-oo:-5) U (-5:5) U (5:+oo)
65/(25-x^2) = 0
x belongs to the empty set
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