Substitution and Input-Output Rules

Substitution and Input-Output Rules

Substitution replaces a letter with a known value. It is the bridge between an algebra rule and an actual answer. Write the value in brackets, especially if it is negative, then follow the order of operations. An input-output table is a useful way to check several substitutions quickly.

Worked example

For $y=4x-3$ and $x=-2$, $y=4(-2)-3=-8-3=-11$. The brackets prevent the negative sign from being lost.

Exam method

1. Copy the rule. 2. Replace each letter with the value in brackets. 3. Calculate in the correct order and check by putting the answer back in the table.