If we have an **algebraic expression**, such as 2x-3 and we are told the value of the **unknown**, e.g. x=7, then we can **evaluate** the expression, that is, find its value, by **substituting** the value of the unknown into the expression. What will the value be in this case?

**Examples**

**Exercise**

Let’s complete exercise 8F from pages 121 and 122 of the textbook:

The answers are below:

**Formulae**

A formula is an equation with a special meaning (e.g. there is a formula for velocity, which is v=u+at). If we know the value of all but one of the unknowns in a formula, we can substitute them into the formula to find the remaining unknown value.

**Examples**

**Exercise**

Let’s complete exercise 8G from page 123 of the textbook:

The answers are below:

**Deriving Formulae**

In the following exercise we derive formulae. This is again like using algebra to solve word problems – we are doing the same mathematics, but using it to **model** reality.

**Exercise**

Let’s complete exercise 8H from pages 124 and 125 of the textbook:

The answers are below: