In algebra, we use a letter to represent an unknown number.
Sometimes we want to substitute a number for the letter in an expression.
Example 1
Evaluate 2a - b when a = 6 and b = 7.
Answer 1
2a - b means (2 × a) - b
So this is (2 × 6) - 7
i.e. 12 - 7 = 5.
Example 2
Work out the value of 5(x - 2) when x = 9.
Answer 2
Substitute x for 9 to get
5(9 - 2) which means 5 × (9 - 2).
Now work out the brackets first to get 9 - 2 = 7.
So this is 5 × 7 = 35