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