## Substitution

Assigning values to a pronumeral is called substitution.

#### Example 2

If $$x$$ = 4, find the value of:
Algebraic expression Solution
5$$x$$ 5 × 4 = 20
$$x$$ + 3 4 + 3 = 7
$$x$$ – 1 4 – 1 = 3
$$\dfrac{x}{2}$$ $$\dfrac{4}{2}$$ = 2

#### Example 3

If $$a$$ = 6 and $$b$$ = 5, find the value of:
Algebraic expression Solution
$$a + b$$ 6 + 5 = 11
$$a\ – b$$ 6 – 5 = 1
10 – $$a$$ 10 – 6 = 4
$$4a\ – 3b$$ 4 × 6 – 3 × 5 = 24 – 15
= 9

#### Example 4

If x = 3, find the value of:
Algebraic expression  Solution
x2 + 1 32 + 1 = 9 + 1
= 10
x2 – 4 362 – 4 = 9 – 4
= 5
4x2 + 2 4 × 32 + 2 = 4 × 9 + 2
= 36 + 2
= 38
2x2 – 4 2 × 32 – 4 = 2 × 9 – 4
= 18 – 4
= 14

#### Example 5

The cost of hiring an electric saw for n hours is (3n + 20) dollars. How much does it cost to hire the saw for 3 hours?

#### Solution

\begin{align}Cost &= 3 × 3 + 20\\ &= \\$29\end{align}