Lời giải:
19.
\(\frac{6^3.9^5}{8.27^4}=\frac{2^3.3^3.(3^2)^5}{2^3.(3^3)^4}=\frac{2^3.3^{13}}{2^3.3^{12}}=3\)
20.
\(\frac{3^{15}.5^{12}}{5^{13}.3^{17}}=\frac{1}{5^{13-12}.3^{17-15}}=\frac{1}{5.3^2}=\frac{1}{45}\)
21.
\(\frac{3^{15}.25^6}{15^{11}.9^4}=\frac{3^{15}.(5^2)^6}{3^{11}.5^{11}.(3^2)^4}=\frac{3^{15}.5^{12}}{3^{19}.5^{11}}=\frac{5}{3^4}=\frac{5}{81}\)