Đáp án:
$C=\dfrac{2^{2020}-1}{3}$
$D=\dfrac{7^{2019}-7}{6}$
Giải thích các bước giải:
$C=1+2^2+2^4+...+2^{2018}$
$4C=2^2+2^4+2^6+...+2^{2020}$
$4C-C=(2^2+2^4+2^6+...+2^{2020})-(1+2^2+2^4+...+2^{2018})$
$3C=2^{2020}-1$
$C=\dfrac{2^{2020}-1}{3}$
$D=7+7^2+7^3+...+7^{2018}$
$7D=7^2+7^3+7^4+...+7^{2019}$
$7D-D=(7^2+7^3+7^4+...+7^{2019})-(7+7^2+7^3+...+7^{2018})$
$6D=7^{2019}-7$
$D=\dfrac{7^{2019}-7}{6}$
