1. Does a formula have to be used when solving problems involving straight line graphs?
The formulas will always work but it is often useful to sketch a diagram and make sure your answers are sensible.
2. Do the formulae for coordinate (straight line) geometry have to be memorised?
It is convenient if they are known off by heart but they are usually given in a test or examination.
3. Should the gradients of straight lines be given as fractions or decimals?
It does not matter what form gradients are given in, but a fraction gives a better indication of the steepness, y-step/x-step, of a line.