@isa
BT3 :
`a`, `2/3` + `a/b` `=` `23/12`
`a/b`` =` `23/12` - `2/3`
`=` `5/4`
`b,` `a/b` - `3/14` `=` `2/7`
`a/b` `=` `2/7` + `3/14`
`=` `1/2`
`c,` `5/6` - `a/b` + `2/3` `=` `4/3`
`3/2` - `a/b` `=` `4/3`
-`a/b` `=` `4/3` - `3/2`
- `a/b` `=` - `1/6`
`a/b` `=` `1/6`