a: \(=2^{16}\cdot2^{20}=2^{36}\)
b: \(=3^{24}\cdot3^{15}\cdot3^{16}=3^{55}\)
\(4^8.2^{20}=\left(2^2\right)^8.2^{20}=2^{16}.2^{20}=2^{16+20}=2^{36}\)
\(9^{12}.27^5.81^4=\left(3^2\right)^{12}.\left(3^3\right)^5.\left(3^4\right)^4=3^{24}.3^{15}.3^{16}=3^{24+15+16}=3^{55}\)