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a in (-oo:+oo)
1/2-(4*a) = -31/2 // + -31/2
1/2-(4*a)-(-31/2) = 0
1/2-4*a+31/2 = 0
16-4*a = 0 // - 16
-4*a = -16 // : -4
a = -16/(-4)
a = 4
a = 4
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