Why does my child get the signs wrong in the quadratic formula?
Your child gets a sign wrong in the quadratic formula, such as using b instead of −b, or b² + 4ac.
Why children do it
The formula has several minus signs, and one is easy to lose, especially with a negative b. Writing each number in brackets, x = (−(b) ± √((b)² − 4(a)(c))) ÷ 2(a), keeps every sign. Putting the solutions back into the equation checks them.
How to help at home
Ask them to write the formula out with brackets round each number before working it out.
The right way
Work out the discriminant, b² − 4ac, of x² + 5x + 6 = 0.
- 5² − 4 × 1 × 6 = 1Here a = 1, b = 5 and c = 6.
- 1It's positive, so there are two solutions.
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
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- Why does my child think x² + 5x + 6 = (x − 2)(x − 3)?
- Why does my child think (x − 2)(x − 3) = 0 gives x = −2 or x = −3?
- Why does my child think x² = 9 only means x = 3?
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- Why does my child think (2x − 3)(x + 4) = 0 gives x = 3?
- Why does my child think y = (x − 2)² + 5 turns at (−2, 5)?
- Why does my child think x² + 6x + 2 = (x + 3)² + 2?