tick mk dc khog a

.....

k co gi

lam gi? trl cau hoi ay ha?

ummm....

1

cho it voiii☹

tick mk dc koo aaa

A=1+4+4mu2+4mu3+...+4mu2021 chia het cho 21

\(= \left(\right. 1 + 4 + 4^{2} \left.\right) + \left(\right. 4^{3} + 4^{4} + 4^{5} \left.\right) . . . + \left(\right. 4^{2019} + 4^{2020} + 4^{2021} \left.\right)\)

\(= \left(\right. 1 + 4 + 4^{2} \left.\right) + 4^{3} \left(\right. 1 + 4 + 4^{2} \left.\right) + . . . + 4^{2019} \left(\right. 1 + 4 + 4^{2} \left.\right)\)

\(= 21 \left(\right. 1 + 4^{3} + . . . + 4^{2019} \left.\right) 21\)

\(= 21 + 21. 4^{3} + . . . + 21. 4^{2019}\)

Do moi so hang cua A deu chia het cho 21 nen A chia het cho 21

Vay A chia het cho 21

k gui anh dc