4 Simplify the three functions as much as possible and verify the result Check on which variables the given functions f1(x), f2(x), and f3(x) depend:
f1(x) = (x1x2 ⊕ x3x4) ∨ x3x4 ∨ x1x2 ∨ x1x3, (4.13)
f2(x) = x2x4 ∨ x1 · ((x3 ⊕ x2) ∨ (x2 · x3)), (4.14)
f3(x) = x1x2 ∨ x3x4 ∨ (x1 ⊕ x4) ∨ (x2 ⊕ x3). (4.15)
Simplify the functions as much as possible and verify the results.