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a in (-oo:+oo)
(-1/2)*a-10 = -25/2 // + -25/2
(-1/2)*a-(-25/2)-10 = 0
(-1/2)*a+25/2-10 = 0
5/2-1/2*a = 0 // - 5/2
-1/2*a = -5/2 // : -1/2
a = -5/2/(-1/2)
a = 5
a = 5
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