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8. (L5.10) Determine whether each of the following are demand functions in the domain where they are positive. (a) x1 = -P1 + 2p2 + m , x2 = 2p1 - 2p2 + m. (b) x1 = (m + p2) / 4p1 , x2 = 3 (m + 2p1) / 4p2 . (c) x1 = (P2 + m) / 2p1 , 22 = (-P2 + m) / 2p2