Propositional and predicate logic

In this example propositional logic assertions are checked using functions to represent the propositions a and b. The following Z3 script checks to see if ${\displaystyle {\overline {a\land b}}\equiv {\overline {a}}\lor {\overline {b}}}$:

(declare-fun a () Bool)
(declare-fun b () Bool)
(assert (not (= (not (and a b)) (or (not a)(not b)))))
(check-sat)

Result:

unsat

Note that the script asserts the negation of the proposition of interest. The unsat result means that the negated proposition is not satisfiable, thus proving the desired result (De Morgan's law).

Solving equations

The following script solves the two given equations, finding suitable values for the variables a and b:

(declare-const a Int)
(declare-const b Int)
(assert (= (+ a b) 20))
(assert (= (+ a (* 2 b)) 10))
(check-sat)
(get-model)

Result:

sat
(model 
  (define-fun b () Int
    -10)
  (define-fun a () Int
    30)
)