183. Hence, if four numbers be proportional, they are also proportional in any other order, provided it be such that similar terms still remain similar. For since, when
| a | = | c | , |
| b | d |
it follows (181) that ad = bc, all the proportions which follow from ad = bc, by the last article, follow also from
| a | = | c | . |
| b | d |
184. From (114) it follows that
| 1 + | a | = | b + a | , |
| b | b |
| and if | a | be | less than | 1, |
| b |
| 1 - | a | = | b - a | , |
| b | b |
| while if | a | be | greater than | 1, | ||
| b | ||||||
| a | - 1 | = | a - b | . |
| b | b |