a)B=(3+32)+(33+34)+.....+(32009+22010)
B=3(1+3)+33(1+3)+......+32009(1+3)
B=3.4 +33.4 +......+32009.4
B=4(3+33+....+32009)chia hết cho 4.
b)B=(3+32+33)+.......+(32008+32009+32010)
B=3(1+3+9)+........+32008(1+3+9)
B=3.13+.....+32008.13
B=13(3+......32008) chia hết cho 13.
Tick cho mình nhé bạn!