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.
Упражнение 3
Can she play chess?
Can he play hockey?
May they go to the cinema?
May I open the window?
How often must children and adults clean their teeth?
Where must children go to?
What must English learners do every day?
What should you keep tidy?
Should children listen to their parent`s advice?
Should they be polite?
Упражнение 4.
1.your brother/ play/ table tennis/ can? – Yes, he…
Can your brother play table tennis? - Yes, he can.
2. your cousin/ can/ ride a horse? – No, he…
Can your cousin ride a horse? - No, he can't.
3.may/ go to/ I / the cinema/ – Yes, you…
May I go to the cinema? - Yes, you may.
4.late/ may/ come home/ she – No, she…
May she come home late? - No, she may not.
5.learn this poem/ must/ she/ by heart? – Yes, she…
Must she learn this poem by heart? - Yes, she must.
6.books/ how many/ they/ must/read? – Only two.
How many books must they read? - Only two.
7.wear/ smart clothes/ for the party/ should/ they? – Yes,…
Should they wear smart clothes for the party? - Yes, they should.
8.salt/ how much/ should/ put in the dish/ I ? – Just a little.
How much salt should I put in this dish? - Just a little.
9.the student/ mobile/ phones/ use/ can/ at the exams? – No, they…
Can the students use mobile phones at the exams? - No, they can't.
10.children/ come to school / in time/ must? – Yes, they…
Must children come to school in time? - Yes, they must
Упражнение 5.
you can't go to the party
He must take his dog for a walk.
I can help you
He mustn't be late
can his brother speak French?
Paul must go there
You mustn't smoke here.
Объяснение:
