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When we substitute a number for a letter, we simply replace the letter with the number.
However, we must remember that 4b means 4 x b.
Example
Work out the value of the following expression when a = 4
3a - 7
Answer
3a - 7
3a just means 3 x a , so we have
3 x a - 7
Change the letter (a) into the number we've been given (4)
3 x 4 - 7
Now multiply first - remember the order of BIDMAS (or BODMAS)
12 - 7 = 5
Sometimes we have a negative number to substitute and then we need to be very careful about the correct signs!
Example
Work out the value of the following expression when a = -4
3a - 7
Answer
3a - 7
3a just means 3 x a , so we have
3 x a - 7
Change the letter (a) into the number we've been given (-4)
3 x -4 - 7
Now multiply first - remember the order of BIDMAS (or BODMAS)
3 x -4 = -12
Finally, subtract 7:
-12 - 7 = -19
When working with negatives, it's important to remember the rules!
Two negative signs next to each other means a positive.
One negative and one positive next to each other means a negative.
If we multiply or divide one positive and one negative, we get a negative.
If we multiply two positives or two negatives, we get a positive.
Are you happy with this?
