S1=0!+1!+2!+3!+4!=34 == 4 mod5
for every n>=5 ; n!==0 mod5
===>
S2=5!+6!+7!+......+64! ==0 mod5
S1+S2 == 4 mod5
(S1+S2 )^2==4^2==1 mod5

View other answers by:Soarer|ali