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a in (-oo:+oo)
(a^2)/2-((7*a)/4) = 15/2 // - 15/2
(a^2)/2-((7*a)/4)-(15/2) = 0
(a^2)/2+(-7/4)*a-15/2 = 0
1/2*a^2-7/4*a-15/2 = 0
DELTA = (-7/4)^2-(-15/2*1/2*4)
DELTA = 289/16
DELTA > 0
a = ((289/16)^(1/2)+7/4)/(1/2*2) or a = (7/4-(289/16)^(1/2))/(1/2*2)
a = 6 or a = -5/2
a in { -5/2, 6 }
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