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x in (-oo:+oo)
s = (n/2)*(a+l) // - (n/2)*(a+l)
s-((n/2)*(a+l)) = 0
s-1*1/2*n*(a+l) = 0
s-1*1/2*n*(a+l) = 0
s-1/2*a*n-1/2*l*n = 0
s-1/2*a*n-1/2*l*n = 0
x belongs to the empty set
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