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D( n )
3*n+15 = 0
3*n+15 = 0
3*n+15 = 0
3*n+15 = 0 // - 15
3*n = -15 // : 3
n = -15/3
n = -5
n in (-oo:-5) U (-5:+oo)
(n+5)/(3*n+15)+(2*n)/3 = 0
(3*(n+5))/(3*(3*n+15))+(2*n*(3*n+15))/(3*(3*n+15)) = 0
3*(n+5)+2*n*(3*n+15) = 0
6*n^2+33*n+15 = 0
6*n^2+33*n+15 = 0
3*(2*n^2+11*n+5) = 0
2*n^2+11*n+5 = 0
DELTA = 11^2-(2*4*5)
DELTA = 81
DELTA > 0
n = (81^(1/2)-11)/(2*2) or n = (-81^(1/2)-11)/(2*2)
n = -1/2 or n = -5
3*(n+5)*(n+1/2) = 0
(3*(n+5)*(n+1/2))/(3*(3*n+15)) = 0
(3*(n+5)*(n+1/2))/(3*(3*n+15)) = 0 // * 3*(3*n+15)
3*(n+5)*(n+1/2) = 0
( n+1/2 )
n+1/2 = 0 // - 1/2
n = -1/2
( n+5 )
n+5 = 0 // - 5
n = -5
n in { -5}
n = -1/2
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