Dưới đây là các phép tính từ câu 11 đến câu 21:

### 11)
\[
\frac{8^6}{27^1} = \frac{(2^3)^6}{3^3} = \frac{2^{18}}{3^3} = 2^{18} \cdot 3^{-3}
\]

### 12)
\[
\frac{4^2 \cdot 5^3}{5^1 \cdot 9^2} = \frac{4^2 \cdot 5^3}{5^1 \cdot (3^2)^2} = \frac{4^2 \cdot 5^{3-1}}{3^4} = \frac{4^2 \cdot 5^2}{3^4} = \frac{(2^2)^2 \cdot (5^1)^2}{3^4} = \frac{2^4 \cdot 5^2}{3^4}
\]

### 13)
\[
\frac{9^{2}}{8^{3}} = \frac{(3^2)^2}{(2^3)^3} = \frac{3^4}{2^9}
\]

### 14)
\[
\frac{21^{9}}{3^{16}} = \frac{(3 \cdot 7)^{9}}{3^{16}} = \frac{3^9 \cdot 7^9}{3^{16}} = 3^{9-16} \cdot 7^9 = 3^{-7} \cdot 7^9
\]

### 15)
\[
\frac{4^{2} \cdot 5^{3}}{5^{1} \cdot 9^{2}} = \frac{(2^{2})^{2} \cdot 5^{3}}{5^{1} \cdot (3^{2})^{2}} = \frac{2^{4} \cdot 5^{3-1}}{3^4} = \frac{2^{4} \cdot 5^{2}}{3^4}
\]

### 16)
\[
\frac{25^{5}}{25^{2} \cdot 8^{2}} = \frac{25^{5-2}}{8^{2}} = \frac{25^{3}}{8^{2}} = \frac{(5^2)^{3}}{(2^3)^{2}} = \frac{5^6}{2^6}
\]

### 17)
\[
\frac{20^{3} \cdot 9^{5}}{20^{5} \cdot 12^{3}} = \frac{20^{3-5} \cdot 9^{5}}{12^{3}} = \frac{20^{-2} \cdot 9^{5}}{12^{3}} = \frac{9^{5}}{20^{2} \cdot 12^{3}}
\]

### 18)
\[
\frac{3^{3} \cdot 25^{5}}{45^{2} \cdot 75^{1}} = \frac{3^{3} \cdot 25^{5}}{(3^{2} \cdot 5)^{2} \cdot (3 \cdot 5^{2})} = \frac{3^{3} \cdot 25^{5}}{3^{4} \cdot 5^{4}} = \frac{3^{3-4} \cdot 25^{5}}{5^{4}} = \frac{3^{-1} \cdot 25^{5}}{5^{4}}
\]

### 19)
\[
\frac{21^{2} \cdot 27^{1}}{32^{2} \cdot 3^{3}} = \frac{(3 \cdot 7)^{2} \cdot (3^{3})^{1}}{(2^5)^{2} \cdot 3^{3}} = \frac{3^{2+3} \cdot 7^{2}}{2^{10} \cdot 3^{3}} = \frac{3^{5-3}}{2^{10}} \cdot 7^{2} = \frac{3^2 \cdot 7^2}{2^{10}}
\]

### 20)
\[
\frac{3^{2} \cdot 33^{1}}{81^{1}} = \frac{3^{2} \cdot 33^{1}}{(3^{4})^{1}} = \frac{3^{2} \cdot 33^{1}}{3^{4}} = 3^{2-4} \cdot 33^{1} = 3^{-2} \cdot 33
\]

### 21)
\[
\frac{45^{2} \cdot 25^{3}}{75^{1}} = \frac{(3^{2} \cdot 5)^{2} \cdot (5^{2})^{3}}{(3 \cdot 5^{2})^{1}} = \frac{3^{4} \cdot 5^{2} \cdot 5^{6}}{3 \cdot 5^{2}} = \frac{3^{4} \cdot 5^{8-2}}{3} = 3^{4-1} \cdot 5^{8} = 3^{3} \cdot 5^{8}
\]

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