1: \(C=1+3^2+3^4+...+3^{2020}\)
=>\(9C=3^2+3^4+...+3^{2022}\)
=>\(9C-C=3^2+3^4+...+3^{2022}-1-3^2-...-3^{2020}\)
=>\(8C=3^{2022}-1\)
=>\(C=\dfrac{3^{2022}-1}{8}\)
2: \(B=1+3+3^2+...+3^{2016}\)
=>\(3B=3+3^2+3^3+...+3^{2017}\)
=>\(3B-B=3+3^2+...+3^{2017}-1-3-...-3^{2016}\)
=>\(2B=3^{2017}-1\)
=>\(B=\dfrac{3^{2017}-1}{2}\)