1) Consider the following statement. How would you express it as a formula of first-order logic?

For all real numbers x and for all real numbers y there is a real number z such that x + y = z.

2) Let the domain of the variable x be . How would you translate into English the expression

xy (x + y = y + x)

What property of the set of real numbers is being expressed?

3) Consider the formula

x (P(x) P(a))

and the interpretation: domain D = {1, 2, 3}, P = {(1, 2), (2, 3), (1, 3)}, a = 1. Is the formula true or false for the given interpretation? Show why.