Решение:

1. $$\frac{1}{\frac{1}{30} + \frac{1}{42}} = \frac{1}{\frac{42+30}{30 \cdot 42}} = \frac{1}{\frac{72}{1260}} = \frac{1260}{72} = \frac{630}{36} = \frac{315}{18} = \frac{105}{6} = \frac{35}{2} = 17.5$$
2. $$\frac{1}{\frac{1}{36} - \frac{1}{44}} = \frac{1}{\frac{44-36}{36 \cdot 44}} = \frac{1}{\frac{8}{1584}} = \frac{1584}{8} = 198$$
3. $$\frac{1}{\frac{1}{36} + \frac{1}{45}} = \frac{1}{\frac{45+36}{36 \cdot 45}} = \frac{1}{\frac{81}{1620}} = \frac{1620}{81} = 20$$
4. $$\frac{1}{\frac{1}{35} - \frac{1}{60}} = \frac{1}{\frac{60-35}{35 \cdot 60}} = \frac{1}{\frac{25}{2100}} = \frac{2100}{25} = 84$$
5. $$\frac{1}{\frac{1}{21} + \frac{1}{28}} = \frac{1}{\frac{28+21}{21 \cdot 28}} = \frac{1}{\frac{49}{588}} = \frac{588}{49} = 12$$
6. $$\frac{1}{\frac{1}{72} - \frac{1}{99}} = \frac{1}{\frac{99-72}{72 \cdot 99}} = \frac{1}{\frac{27}{7128}} = \frac{7128}{27} = 264$$

Ответы:

• 1) 17.5
• 2) 198
• 3) 20
• 4) 84
• 5) 12
• 6) 264