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.
1) My sister is tall and she has dark hair and blue eyes.
2) I don't believe in fortune tellers.
3) She is practising her piano lessons right now.
4) We walk to work every day.
5) Tom is a lazy boy. He gets up at 12every day.
6) I like pasta very much.
7) My grandma enjoys visiting her grandchildren.
8) They practise their piano lessons in the living room.
9) They play chess at weekends.
10) We don't believe in miracles.
11) He washes his own clothes.
12) Peter doesn't smoke too much.
13) My grandfather doesn't know anything about music.
14) Ernest has a shop. He sells pets.
15) I always wear comfortable clothes.
16) Mum is washing the dishes now. She always washes the dishes after every meal.
17) They are having a bath now.
18) We don't do the homework after class.