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a in (-oo:+oo)
72 = 4*(1/2)*a // - 4*(1/2)*a
72-(4*(1/2)*a) = 0
72-2*a = 0
72-2*a = 0 // - 72
-2*a = -72 // : -2
a = -72/(-2)
a = 36
a = 36
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