Задание 3. Найдите значение выражения: 1) 1/(1/30 + 1/42); 2) 1/(1/36 − 1/44); 3) 1/(1/36 + 1/45); 4) 1/(1/35 − 1/60); 5) 1/(1/21 + 1/28); 6) 1/(1/72 − 1/99); 7) 10·(1/5)^2 − 12·1/5; 8) 8·(1/4)^2 − 14·1/4; 9) 21·(1/7)^2 − 10·1/7; 10) 6·(1/3)^2 − 17·1/3; 11) 18·(1/9)^2 − 20·1/9; 12) 15·(1/5)^2 − 8·1/5.

1. \(\frac{1}{\frac{1}{30}+\frac{1}{42}}=\frac{1}{\frac{7+5}{210}}=\frac{210}{12}=\frac{35}{2}\).
2. \(\frac{1}{\frac{1}{36}-\frac{1}{44}}=\frac{1}{\frac{44-36}{1584}}=\frac{1584}{8}=198\).
3. \(\frac{1}{\frac{1}{36}+\frac{1}{45}}=\frac{1}{\frac{5+4}{180}}=20\).
4. \(\frac{1}{\frac{1}{35}-\frac{1}{60}}=\frac{1}{\frac{60-35}{2100}}=84\).
5. \(\frac{1}{\frac{1}{21}+\frac{1}{28}}=\frac{1}{\frac{4+3}{84}}=12\).
6. \(\frac{1}{\frac{1}{72}-\frac{1}{99}}=\frac{1}{\frac{99-72}{7128}}=264\).
7. \(10\cdot\left(\frac15\right)^2-12\cdot\frac15=\frac{10}{25}-\frac{12}{5}=\frac25-\frac{12}{5}=-2\).
8. \(8\cdot\left(\frac14\right)^2-14\cdot\frac14=\frac{8}{16}-\frac{14}{4}=\frac12-\frac72=-3\).
9. \(21\cdot\left(\frac17\right)^2-10\cdot\frac17=\frac{21}{49}-\frac{10}{7}=\frac37-\frac{10}{7}=-1\).
10. \(6\cdot\left(\frac13\right)^2-17\cdot\frac13=\frac{6}{9}-\frac{17}{3}=\frac23-\frac{17}{3}=-5\).
11. \(18\cdot\left(\frac19\right)^2-20\cdot\frac19=\frac{18}{81}-\frac{20}{9}=\frac29-\frac{20}{9}=-2\).
12. \(15\cdot\left(\frac15\right)^2-8\cdot\frac15=\frac{15}{25}-\frac85=\frac35-\frac85=-1\).

Ответ: 1) \(\frac{35}{2}\); 2) \(198\); 3) \(20\); 4) \(84\); 5) \(12\); 6) \(264\); 7) \(-2\); 8) \(-3\); 9) \(-1\); 10) \(-5\); 11) \(-2\); 12) \(-1\).