Найти вторые частные производные указанных функций. Убедиться в том, что  Z"xy=Z"ух

z=arccos(x-5y)

Time Expressions

Exercise 1. Translate the time idioms from English into Russian. Learn them.

save time; kill time; work against the clock; in the nick of time; out of time; a waste of time; ahead of time; behind one's time

Exercise 2. Match the time idioms from Exercise I with the meanings given below.

1. You arrive or finish something before it is too late.

2. You do something the quick way.

3. You have no more time to do something.

4. You do something that is not so interesting or important to pass

time.

5. You do something pointless or useless.

6. You are forward-looking. 7You are old-fashioned,

8. You work hard because you know that you have a deadline.