Why does my child think 2x + 5 is 11 when x = −3?
Your child leaves out the minus sign when putting a negative number in place of a letter, such as 2x + 5 with x = -3 as 11.
Why children do it
Putting −3 in place of x, the minus sign is easy to leave behind, giving 2 × 3 + 5 = 11. Writing the negative number in brackets, 2 × (−3) + 5, keeps it: −6 + 5 is −1.
How to help at home
Ask them to write the negative number in brackets when they put it in: 2 × (-3).
The right way
When x = −9, work out: 5x + 10
- 5 × (−9) + 10Put -9 in place of x, in brackets.
- −45 + 105 times -9 is -45.
- −35Then add 10.
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