Al , Cr , Ni + HNO3(конц.;разб.)

Подскажите пожалуйста,откуда берутся сверху и снизу +1,-2 и т.д  и как подставлять перед элементом цифры?

В ромбе  ABCD  диагонали пересекаются в точке O , угол A =31 градус. Найдите  углы треугольника BOC

заполнить пропуски артиклями,где необходимо:

1 I am ... engineer of Avtoexport.

2   Avtoexport. does business with ... lot of ... countries.

3 We sell cars,trucks and other machines to different countrics of ... world.

4 Our office is on ... fifth floor.

5 It is ... large room with three windows in it.

6 There are ... seven desks in office with ... telephone on every desk.

7 You can also see four book-cases in  our room. There are ... lot of ... letters, ... 

catalogues and Journals in them.

8 Many foreign businessmen come to ... our office every day. But we do not receive 

them in this room. There is another room for it.

9 Our  engineer discuss ... pricess,  ... terms of  payment and delivery and ... lot of other questions with them. All our  engineer know one or two foreign languages and they can have talks with foreign businessmen in languages.