a)S=30+32+...+32002=1+32+...+32002
=>32.S=32+34+...+32004
=>9S=32+34+...+32004
=>9S-S=(32+34+...+32004)-(1+32+...+32002)
=>8S=32004-1
=>S=\(\frac{3^{2004}-1}{8}\)
b)S=30+32+...+32002=1+32+...+32002
=(1+32+34)+...+(31998+32000+32002)
=91+....+31998.91
=91.(1+...+31998)
=7.13.(1+...+31998) chia hết cho 7
Vậy S chia hết cho 7