Don’t Be Negative – A Common GCSE Maths Mistake!

May 11, 2014 § Leave a comment

Okay so this might be a bit of a boring blog post! But, with the GCSE exams round the corner, I thought I would highlight one of the most common mistakes seen when marking GCSE maths papers.

That problem is negative numbers. They just get students in a right old pickle! And that’s why they crop up every year in the exam papers! But they won’t be obvious . . . . they are often very sneaky and catch people off guard. For instance, you will see minus signs when expanding brackets or in substitution or indices questions.

So, here are the simple rules:

MINUS X MINUS = PLUS

MINUS X PLUS = MINUS

Take this past paper question as an example:

V = 3b + 2b2

Find the value of V when b = –4

First, substitute the number into the equation.

3b + 2b² =   3 x -4 + 2 x -4²

3 x -4 is PLUS x MINUS, so it becomes a MINUS: 3 x -4 = -12

2 x -4² is a bit more complicated because we need to do the squaring first:
-4² = -4 x -4 which is MINUS x MINUS, so it becomes a PLUS. -4 x -4 = 16

2 x 16 = 32

So, 3b + 2b² =   3 x -4 + 2 x -4² = -12 + 32 = 20

Just take it step by step and CHECK YOUR WORK! We wish everyone doing the GCSE exams all the best.

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