27. Find the least positive residues of
a) 6! modulo 7
b) 10 ! modulo 11
c) 12! modulo 13
d) 16! modulo 17 .
e) Can you propose a theorem from the above congruences?
27. a) -1
b) -1
c) -1
d) -1
e) $(p-1)!\equiv-1(\bmod p)$ when $p$ is prime