## How to use this online book

In the desktop-computer version of this book, icons appear at the top of each page. On other devices, all or some of these icons may also appear.

• Navigation: Navigate between pages using the arrows.

• Words can be searched for using Search button on the menu:

• The font can be changed (e.g., enlarged) using the Font button on the menu:

1. What is $$1+1$$?
2. What is $$1+2$$?
3. Is $$2 + 2=4$$?