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D( x )
x^2-16 = 0
x^2-16 = 0
x^2-16 = 0
1*x^2 = 16 // : 1
x^2 = 16
x^2 = 16 // ^ 1/2
abs(x) = 4
x = 4 or x = -4
x in (-oo:-4) U (-4:4) U (4:+oo)
(-12*x)/(x^2-16) = 0
-12*x = 0 // : -12
x = 0
x = 0
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