# Year 9

## Number and Algebra

### Indices

Index notation is comparatively modern. The Greek writer Diophantus used the symbol $$\triangle^T$$ to denote what we would call x² and used K for x³. In medieval algebra, Q and C were used instead. Even in the 16th century, people were still writing xxx for x³. The modern notation didn't appear in texts until late in the 18th century. It is an extremely useful and efficient notation.