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a in (-oo:+oo)
(1/2)*a = 3.1 // - 3.1
(1/2)*a-3.1 = 0
1/2*a-3.1 = 0 // + 3.1
1/2*a = 3.1 // : 1/2
a = 3.1/1/2
a = 6.2
a = 6.2
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