GCSE Formulae To Remember

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So we are almost at exam season now, and a common thing that I have been seeing in my students relates to remembering certain formulae. If I ask someone to give me the quadratic formula for instance, they can often write correctly:

 x = \frac{-b \pm \sqrt{b^2-4ac}}{2a}.

The problem comes if I then ask what equation this formula is the solution to. Quite a few students struggle to give me the equation

 ax^2 + bx + c = 0.

The form of this equation is really important for correct application of the quadratic formula. In particular, it is important to group all the terms together and rearrange so that it equals zero on one side. For example, a sneaky trick would be for a question in the exam to ask for a solution to  x^2 - 2x + 1 = 1. The quadratic on the left hand side is an easy one and students should be able to factorize it, but failing to notice that it equals one on the right might lead to the wrong answer x=1. By remembering the form that quadratic equations have to be put into before they can be solved, one can see that the correct answer is x=0 \text{ or } 2.

Another one that causes problems is the trigonometry mnemonic SOH CAH TOA. Often a student can say that they need to apply 'SOHCAHTOA' to a problem, but when I ask them to write down the equations that it refers to, again they often struggle. A mnemonic is only useful if the student can actually remember the equations that it refers to.

The formulae that need to be memorized for Edexcel GCSE can be found here.

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