ExpertMedium2 marksShort Answer
AQA GCSE · Question 12.1 · Algebra
Work out the value of x² + 7x when x = -4
Work out the value of x² + 7x when x = -4
How to approach this question
1. Substitute x = -4 into the expression x² + 7x.
2. Remember to use brackets when substituting a negative number, especially when squaring: (-4)² + 7(-4).
3. Calculate (-4)². This is (-4) * (-4) = 16.
4. Calculate 7(-4). This is 7 * (-4) = -28.
5. Combine the results: 16 + (-28), which is 16 - 28.
6. Calculate the final answer: 16 - 28 = -12.
Full Answer
-12
We need to substitute x = -4 into the expression x² + 7x.
It's good practice to use brackets when substituting to avoid errors with negative numbers.
Expression: (x)² + 7(x)
Substitute x = -4:
(-4)² + 7(-4)
Now, calculate each part:
(-4)² = (-4) × (-4) = 16 (A negative number squared becomes positive)
7(-4) = 7 × -4 = -28
Now, combine the two parts:
16 + (-28) = 16 - 28 = -12.
The value of the expression is -12.
Common mistakes
✗ Calculating (-4)² as -16. This is the most common mistake. (-4)² = 16, whereas -4² = - (4*4) = -16.
✗ Making a sign error with 7(-4), getting +28 instead of -28.
✗ Incorrectly adding/subtracting the final numbers, e.g., 16 - 28 = 12.
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