9 Assuming that our proof calculus for predicate logic is sound, show that the validity of the following sequents cannot be proved by finding for each sequent a model such that all formulas to the left of evaluate to T and the sole formula to the right of evaluates to F a xyS(x,y) yxS(x,y) b xP(x), yQ(y)z(P(z)Q(z)) c x(P(x)Q(x))x(P(x)Q(x))

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9  Assuming that our proof calculus for predicate logic is sound, show that the validity of the following sequents cannot be proved by finding for each sequent a model such that all formulas to the left of evaluate to T and the sole formula to the right of evaluates to F a  xyS(x,y) yxS(x,y) b  xP(x), yQ(y)z(P(z)Q(z)) c  x(P(x)Q(x))x(P(x)Q(x))

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Question: 9. Assuming that our proof calculus for predicate logic is sound, show that the validity of the following sequents cannot be proved by finding for

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