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n in (-oo:+oo)
(n/2)*(n+1) = 10 // - 10
(n/2)*(n+1)-10 = 0
1/2*n*(n+1)-10 = 0
1/2*n*(n+1)-10 = 0
1/2*n^2+1/2*n-10 = 0
1/2*n^2+1/2*n-10 = 0
1/2*n^2+1/2*n-10 = 0
DELTA = (1/2)^2-(-10*1/2*4)
DELTA = 81/4
DELTA > 0
n = ((81/4)^(1/2)-1/2)/(1/2*2) or n = (-(81/4)^(1/2)-1/2)/(1/2*2)
n = 4 or n = -5
(n+5)*(n-4) = 0
(n+5)*(n-4) = 0
( n+5 )
n+5 = 0 // - 5
n = -5
( n-4 )
n-4 = 0 // + 4
n = 4
n in { -5, 4 }
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