Đây nè bạn. Bạn nhớ tick hộ mik nha:
\(C=1+4+4^2+4^3+...+4^{2017}\)
\(4C=4.\left(1+4+4^2+4^3+...+4^{2017}\right)\)
\(4C=4+4^2+4^3+4^4+...+4^{2018}\)
\(3C=\left(4+4^2+4^3+4^4+...+4^{2018}\right)-\left(1+4+4^2+4^3+...+4^{2017}\right)\)
\(3C=4^{2018}-1\)
\(C=\dfrac{4^{2018}-1}{3}\)