Dưới đây là các bước giải các bài toán trong hình:

**2. Thực hiện phép tính (bằng cách hợp lí nếu có thể):**

a)
\[ \frac{8}{21} - 1 - \frac{3}{14} - \left( -\frac{13}{21} \right) + 0,25 \]

- Quy đồng mẫu số:
\[ \frac{8}{21} - 1 - \frac{3}{14} + \frac{13}{21} + 0,25 \]
\[ = \frac{8}{21} + \frac{13}{21} - 1 - \frac{3}{14} + 0,25 \]
\[ = \frac{21}{21} - 1 - \frac{3}{14} + 0,25 \]
\[ = 1 - 1 - \frac{3}{14} + 0,25 \]
\[ = - \frac{3}{14} + 0,25 \]
\[ = - \frac{3}{14} + \frac{1}{4} \]
\[ = - \frac{3}{14} + \frac{7}{28} \]
\[ = - \frac{3}{14} + \frac{2}{7} \]
\[ = - \frac{3}{14} + \frac{4}{14} \]
\[ = \frac{1}{14} \]

b)
\[ \frac{5}{8} \cdot \frac{2}{5} = \frac{10}{40} = \frac{1}{4} \]
\[ \frac{5}{8} \cdot \frac{1}{3} = \frac{5}{24} \]

c)
\[ \frac{4}{9} - \frac{3}{5} + \frac{1}{5} + \left( \frac{5}{9} + \frac{1}{5} \right) : \frac{1}{5} \]
\[ = \frac{4}{9} - \frac{3}{5} + \frac{1}{5} + \left( \frac{5}{9} + \frac{1}{5} \right) \cdot 5 \]
\[ = \frac{4}{9} - \frac{3}{5} + \frac{1}{5} + \left( \frac{5}{9} + \frac{1}{5} \right) \cdot 5 \]
\[ = \frac{4}{9} - \frac{3}{5} + \frac{1}{5} + \left( \frac{5}{9} \cdot 5 + \frac{1}{5} \cdot 5 \right) \]
\[ = \frac{4}{9} - \frac{3}{5} + \frac{1}{5} + \left( \frac{25}{9} + 1 \right) \]
\[ = \frac{4}{9} - \frac{3}{5} + \frac{1}{5} + \left( \frac{25}{9} + \frac{9}{9} \right) \]
\[ = \frac{4}{9} - \frac{3}{5} + \frac{1}{5} + \frac{34}{9} \]
\[ = \frac{4}{9} + \frac{34}{9} - \frac{3}{5} + \frac{1}{5} \]
\[ = \frac{38}{9} - \frac{2}{5} \]
\[ = \frac{38 \cdot 5}{9 \cdot 5} - \frac{2 \cdot 9}{5 \cdot 9} \]
\[ = \frac{190}{45} - \frac{18}{45} \]
\[ = \frac{172}{45} \]

d)
\[ \frac{6}{7} : \left( \frac{3}{26} + \frac{3}{13} \right) \cdot \frac{6}{7} : \left( \frac{1}{10} + \frac{8}{5} \right) \]
\[ = \frac{6}{7} : \left( \frac{3}{26} + \frac{6}{26} \right) \cdot \frac{6}{7} : \left( \frac{1}{10} + \frac{16}{10} \right) \]
\[ = \frac{6}{7} : \left( \frac{9}{26} \right) \cdot \frac{6}{7} : \left( \frac{17}{10} \right) \]
\[ = \frac{6}{7} \cdot \frac{26}{9} \cdot \frac{6}{7} \cdot \frac{10}{17} \]
\[ = \frac{6 \cdot 26 \cdot 6 \cdot 10}{7 \cdot 9 \cdot 7 \cdot 17} \]
\[ = \frac{9360}{749} \]

**3. Thực hiện phép tính:**

a)
\[ \frac{5^4 \cdot 20^4}{25^5 \cdot 45} \]
\[ = \frac{(5^4 \cdot 2^4 \cdot 10^4)}{(5^2 \cdot 5^3 \cdot 45)} \]
\[ = \frac{(5^4 \cdot 2^4 \cdot 10^4)}{(5^5 \cdot 45)} \]
\[ = \frac{(5^4 \cdot 2^4 \cdot 10^4)}{(5^5 \cdot 45)} \]
\[ = \frac{(5^4 \cdot 2^4 \cdot 10^4)}{(5^5 \cdot 45)} \]

b)
\[ \frac{4^3 \cdot 25^5 \cdot 9^3}{82 \cdot 125^3 \cdot 35} \]
\[ = \frac{(4^3 \cdot 25^5 \cdot 9^3)}{(82 \cdot 125^3 \cdot 35)} \]

c)
\[ \frac{6^3 + 3.6^2 + 3^3}{-13} \]
\[ = \frac{(6^3 + 3.6^2 + 3^3)}{-13} \]

**4. Tìm x, biết:**

a)
\[ x - \frac{3}{4} = \frac{2}{7} \]
\[ x = \frac{2}{7} + \frac{3}{4} \]
\[ x = \frac{8}{28} + \frac{21}{28} \]
\[ x = \frac{29}{28} \]

b)
\[ -\frac{3}{8} x - 0,75 = -\frac{1}{2} \]
\[ -\frac{3}{8} x = -\frac{1}{2} + 0,75 \]
\[ -\frac{3}{8} x = -\frac{1}{2} + \frac{3}{4} \]
\[ -\frac{3}{8} x = \frac{1}{4} \]
\[ x = -\frac{1}{4} \cdot \frac{8}{3} \]
\[ x = -\frac{2}{3} \]

c)
\[ (0,25 - x) : \frac{3}{5} = -\frac{3}{4} \]
\[ 0,25 - x = -\frac{3}{4} \cdot \frac{5}{3} \]
\[ 0,25 - x = -\frac{5}{4} \]
\[ x = 0,25 + \frac{5}{4} \]
\[ x = \frac{1}{4} + \frac{5}{4} \]
\[ x = \frac{6}{4} \]
\[ x = \frac{3}{2} \]

d)
\[ \frac{3}{5} (4x - 1,2) = -\frac{12}{25} \]
\[ 4x - 1,2 = -\frac{12}{25} \cdot \frac{5}{3} \]
\[ 4x - 1,2 = -\frac{4}{5} \]
\[ 4x = -\frac{4}{5} + 1,2 \]
\[ 4x = -\frac{4}{5} + \frac{6}{5} \]
\[ 4x = \frac{2}{5} \]
\[ x = \frac{2}{5} \cdot \frac{1}{4} \]
\[ x = \frac{1}{10} \]