1. Chứng minh :
a) 10^28 + 8 chia hết cho 72
b) 481^n + 1999^1999 chia hết cho 10
2. Tìm x :
a)\(\frac{x+29}{31}\)- \(\frac{x+27}{33}\)= \(\frac{x+17}{43}\)- \(\frac{x+15}{45}\)
b) \(\frac{1909-x}{91}\)+ \(\frac{1907-x}{93}\)+\(\frac{1905-x}{95}\)+ \(\frac{1903-x}{97}\)+4 = 0