Students will see how the difference formula for tan can be used to find the acute angle between two lines, firstly, when neither line is vertical, then when exactly one line is vertical.

Watch these Eddie Woo videos:

Conclude by giving your students these challenges:

- 2-digit Square by NRICH
- What's Possible? by NRICH
- Pegboard Quads by NRICH
- Factorising with Multilink by NRICH

In the figure below, the radius of the smallest circle is \(1.\) What is the radius of the largest circle?

Here's the solution.