1991≡−1(mod12)⇒19911993≡−1(mod12);1993≡1(mod12)⇒19931991≡1(mod12).19911993+19931991⋮12.1991≡−1(mod12)⇒
19911993≡−1(mod12);1993≡1(mod12)⇒19931991≡1(mod12).19911993+19931991⋮12.1991≡−1(mod12)⇒
19911993≡−1(mod12);1993≡1(mod12)⇒19931991≡1(mod12).19911993+19931991⋮12.