решение. 1. ri=2*r; rii=r/2; ri/rii=4; не надо ни о какой площади сечения говорить. два куска проволоки содинили последовательно, получили ri=2*r; два куска той же проволоки соединили параллельно, получили rii=r/2; ну, а теперь раздели первое на второе!
а теперь общий случай: ri=r*n; rii=r/n; ri/rii=n^2; кусок проволоки разрезали на n одинаковых кусков. сначала эти куски соединяют последовательно: ri=r*n; затем соединяют параллельно rii=r/n; ну и дели первое на второе и делай выводы.
The Moscow Zoo is the _oldest_ in Russia. It was opened in 1864. It is also the _biggest_. It covers the territory of 22 hectares. It is situated in the centre of Moscow, near the Garden Ring. Today the Zoo is home to 1,000 species and _more_ than 6,000 animals.
The collection includes animals from all over the world: giraffes and elephants, bears and tigers, dolphins and flamingoes. For many animals, cages are replaced by enclosures that recreate animals' natural environment. This gives a _better_ lifestyle to the animals and a _more realistic_ experience to the visitors.
The Zoo also has beautiful gardens as well as innovative zoo buildings which house many animals in winter.
1. must
2. have
3. may
4. can
5. should
6. need