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D( n )
2*n+6 = 0
2*n+6 = 0
2*n+6 = 0
2*n+6 = 0 // - 6
2*n = -6 // : 2
n = -6/2
n = -3
n in (-oo:-3) U (-3:+oo)
(4*n)/(2*n+6) = 0
4*n = 0 // : 4
n = 0
n = 0
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