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a in (-oo:+oo)
2/5-((1/2)*a) = -51/10 // + -51/10
2/5-((1/2)*a)-(-51/10) = 0
(-1/2)*a+2/5+51/10 = 0
11/2-1/2*a = 0 // - 11/2
-1/2*a = -11/2 // : -1/2
a = -11/2/(-1/2)
a = 11
a = 11
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