Obs. 1*2*3*...*n = n! .
a). 6!*13-5!*12= \
= (1*2*3*4*5*6)*13 - (1*2*3*4*5)*12=5!*(6*13-12)=
= 5!*66 = 7920 ;
b). 5! - 1! - 2! - 3! - 4! =
=(1*2*3*4*5)-(1)-(1*2)-(1*2*3)-(1*2*3*4)=120-1-2-6-24=
= 120 - 33 = 87 ;
c). 7!-6!+5!-4! =
= (1*2*3*4*5*6*7)-(1*2*3*4*5*6)+(1*2*3*4*5)-(1*2*3*4) =
= 5040 - 720 + 120 - 24 = 4416 ;