\(A=2^0+2^1+2^2+-.+2^{1990}\)
\(2A=2\left(2^0+2^1+2^2+-.+2^{1990}\right)\)
\(2A=2^1+2^2+2^3+-.+2^{1991}\)
\(2A-A=\left(2^1+2^2+2^3+-.+2^{1991}\right)-\left(2^0+2^1+2^2+-.+2^{1990}\right)\)
\(A=2^{1991}-2^0=2^{1991}-1\)
\(B=a^0+a^1+a^2+a^3+-.+a^n\)
\(B.a=a^1+a^2+a^3+a^4+-.+a^{n+1}\)
\(B.a-B=\left(a^1+a^2+a^3+a^4+-..+a^{n+1}\right)-\left(a^0+a^1+a^2+a^3+-.+a^n\right)\)
\(B.a=a^{n+1}-1\Leftrightarrow B=\dfrac{a^{n+1}-1}{a}\)
\(C=1+3+3^2+-.+3^{50}\)
\(3C=3\left(1+3+3^2+-.+3^{50}\right)\)
\(3C=3+3^2+3^3+-.+3^{51}\)
\(3C-C=\left(3+3^2+3^3+-.+3^{51}\right)-\left(1+3+3^2+-.+3^{50}\right)\)
\(2C=3^{51}-1\Rightarrow C=\dfrac{3^{51}-1}{2}\)
