When we substitute a number for a letter, we simply replace the letter with the number.
However, we must remember that 4b means 4 × b and 4b2 means 4 × b × b.
Example
Work out the value of the following expression when a = 4 and b = 9
3a2 - 7 - 2√ab
Answer
= 3 × a × a - 7 - 2 × √a × b
= 3 × 4 × 4 - 7 - 2 × √4 × 9
= 48 - 7 - 2 × √36
= 48 - 7 - 2 × 6
= 48 - 7 - 12
= 29
Phew - there's a lot to do, but take it slowly, one step at a time and you'll get there!
Let's get started!
View the activity introduction for more information on the topic
Read the question aloud
Open the accessibility toolbar to change fonts and contrast, choose a different language, use a ruler and more
Question
Do you want to skip questions and finish?
Hold up, it looks like there may be an issue with your spelling. Why not try it again?
You need to check your answer before you proceed. Are you sure you want to skip?
Teacher explanation