вычисление выражений: а) $$\frac{16}{81} : \frac{15}{57} - \frac{19}{4}$$ б) $$\frac{72}{49} - \frac{5}{88} : \frac{77}{25}$$ в) $$\frac{18}{95} - \frac{35}{3} \cdot \frac{19}{42}$$ г) $$\frac{20}{23} - \frac{78}{15} \cdot \frac{16}{73}$$

Решение:

1. $$\frac{16}{81} : \frac{15}{57} - \frac{19}{4} = \frac{16}{81} \cdot \frac{57}{15} - \frac{19}{4} = \frac{16 \cdot 57}{81 \cdot 15} - \frac{19}{4} = \frac{16 \cdot 19}{27 \cdot 15} - \frac{19}{4} = \frac{304}{405} - \frac{19}{4} = \frac{1216 - 7695}{1620} = -\frac{6479}{1620} \approx -3.999$$
2. $$\frac{72}{49} - \frac{5}{88} : \frac{77}{25} = \frac{72}{49} - \frac{5}{88} \cdot \frac{25}{77} = \frac{72}{49} - \frac{125}{6776} = \frac{72 \cdot 138.28 - 125}{6776} \approx \frac{9956.16 - 125}{6776} \approx \frac{9831.16}{6776} \approx 1.45$$
3. $$\frac{18}{95} - \frac{35}{3} \cdot \frac{19}{42} = \frac{18}{95} - \frac{35 \cdot 19}{3 \cdot 42} = \frac{18}{95} - \frac{665}{126} = \frac{18 \cdot 126 - 665 \cdot 95}{95 \cdot 126} = \frac{2268 - 63175}{11970} = -\frac{60907}{11970} \approx -5.088$$
4. $$\frac{20}{23} - \frac{78}{15} \cdot \frac{16}{73} = \frac{20}{23} - \frac{78 \cdot 16}{15 \cdot 73} = \frac{20}{23} - \frac{1248}{1095} = \frac{20 \cdot 1095 - 1248 \cdot 23}{23 \cdot 1095} = \frac{21900 - 28704}{25185} = -\frac{6804}{25185} \approx -0.27$$

Ответ: $$-3.999$$, $$1.45$$, $$-5.088$$, $$-0.27$$.