I know the formula, but I don't understand it.

A thorough understanding of all the mathematical formulas prescribed in the subject specification is crucial for your survival in the no-mans-land of the maths exam, where your examiner tries his best to murder your grade using the tactical warfares of disguise, manipulate and misdirect.

However, people's minds work differently, and for subjects like maths, sometimes it really is just a simple logic that your brain has failed to capture. But to capture such a logic by yourself, or even simply understanding where it is you've gone wrong is a long and tedious process that eats away at your mind. As a result, it is a lot easier to take for granted the formula that you are taught and just mindlessly follow it until you are blown apart by the well-disguised landmines placed in the exam paper.

To cope with this problem, I recommend practising large amounts of 'application questions'. These questions take the abstract underlying logic of the formula and show it logically to you by using everyday examples. Maths is derived from real life after all, and the logic of real-life events is what composes the mathematical formulas in the very end.

Practising these questions will help you to better apply the mathematical formulas and to identify the 'abnormalities' in a complicated exam question, clearing you a path through all the distractions and messiness, towards the correct answer.