Ta có: a ≡ a (mod 5) a -a ≡ a - a (mod 5) a - a ≡ 0 (mod 5)Vậy a - a 30 Cách
2: a - a = a( a -1) = a[ (a) - 1 ] = a(a -1)(a +1) = a(a-1)(a+1)(a -4+5) = a(a-1)(a+1)[ ( a -
2)(a+2)+5] = a(a- 1)(a+1)(a-
2)(a+2)+5a(a-1)(a+1) Nếu a=0 thì a - a = 0 30Nếu a>0 thì a(a- 1)(a+[r]