Why does my child get the signs wrong taking one fraction from another?
Your child takes a minus in front of a fraction to only the first term on top, such as −(x − 2) as −x − 2 instead of −x + 2.
Why children do it
The minus sign sits in front of the whole fraction, but it's easy to apply it to the first term on top and leave the rest. In (x + 4)/3 − (x − 2)/5, the second top is taken away whole: −(x − 2) is −x + 2. Putting the top in a bracket before taking it away keeps every sign right.
How to help at home
Ask them to put the top in a bracket before taking it away: −(x − 2). Then the minus multiplies both terms.
The right way
Simplify: (x + 4)/3 − (x − 2)/5
- (5(x + 4) − 3(x − 2))/15Write both over 15, with the second top in a bracket.
- (5x + 20 − 3x + 6)/15The minus changes both signs: -3 × (-2) is +6.
- (2x + 26)/15Collect like terms.
Find out where your child is
A free maths check, in about 15 minutes, shows what your child knows and the mistakes they make. No sign-up.
Start the free checkRelated mistakes
- Why does my child think 3x is 35 when x = 5?
- Why does my child think 2x + 5 is 11 when x = −3?
- Why does my child think 3x + 4 = 7x?
- Why does my child think 7x − 2 − 3x + 4 = 10x + 2?
- Why does my child think 5(x + 4) = 5x + 4?
- Why does my child think −2(x − 3) = −2x − 6?
- Why does my child think 6x + 9 = 3(2x + 9)?
- Why does my child think (6x + 9)/3 = 2x + 9?