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n in (-oo:+oo)
(n/2)*(n-1) = 45 // - 45
(n/2)*(n-1)-45 = 0
1/2*n*(n-1)-45 = 0
1/2*n*(n-1)-45 = 0
1/2*n^2-1/2*n-45 = 0
1/2*n^2-1/2*n-45 = 0
1/2*n^2-1/2*n-45 = 0
DELTA = (-1/2)^2-(-45*1/2*4)
DELTA = 361/4
DELTA > 0
n = ((361/4)^(1/2)+1/2)/(1/2*2) or n = (1/2-(361/4)^(1/2))/(1/2*2)
n = 10 or n = -9
(n+9)*(n-10) = 0
(n+9)*(n-10) = 0
( n+9 )
n+9 = 0 // - 9
n = -9
( n-10 )
n-10 = 0 // + 10
n = 10
n in { -9, 10 }
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