Substitute positive values
Objective. Evaluate expressions by replacing variables with numbers.
- 1. Learn
- 2. Worked example
- 3. Practice
- 4. Feedback
- 5. Continue
Learn: the key idea
Substitution means replacing each variable in an expression with a given number and then working out the value. To substitute into 2x+5 when x=4, replace x with 4 to get 2×4+5, which equals 13. Following the correct order of operations matters: multiplication and powers are worked out before addition or subtraction. For an expression with two variables like 3a+2b, substitute both values together, such as a=2 and b=5 giving 3×2+2×5=16.
Worked example
Find the value of 4x² − 3x when x=3 by substitution.
- 1Substitute x=3 into the expression: 4×3²−3×3.
- 2Work out the power first: 3²=9.
- 3Multiply: 4×9=36 and 3×3=9.
- 4Subtract: 36−9=27.
Answer: 27
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Start practisingCommon mistake. Pupils sometimes add the variable's value to the coefficient instead of multiplying, treating 5x as 5+x. Remember a coefficient sitting next to a variable always means multiplication.
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