a) 3 divide pe (3a+6b+9c)
3a+6b+9c=3(a+2b+3c) deci divide 3 acest numar
b) 10 divide (9 l^n +9 ^ n+1), n apartine N
9 ^ n +9 ^ n+1=9 ^n(1+9)=9^ n*10 deci divide 10 acest numar
c) 7 divide (1001+2002+...+9009)
1001+2002+...+9009=1001(1+2+...+9)=1001*9*10:2=1001*45=45045=7*6435 deci divide 7 acest numar
d) 11 divide (2 ^44 +2^47+ 2^45)
2 ^ 44 +2 ^47+ ^ 45=2^44(1+2^3+2)=2^44*11 deci div 11 acest numar