Wrong move:

Forgetting to state reasons for angle answers

Why:

CIE awards up to 50% of marks for reasoning, so you lose half the points even if your numerical answer is correct

Correct move:

Write the full standard reason for every angle calculation, e.g. 'angles on a straight line sum to 180°'

Wrong move:

Using parallel line angle rules for non-parallel lines

Why:

Alternate, corresponding and co-interior rules only apply if lines are explicitly marked as parallel or stated as parallel in the question

Correct move:

Only use these rules if you see the parallel line symbol ($\parallel$) on the diagram or the question confirms lines are parallel

Wrong move:

Dividing the total interior angle sum by n for irregular polygons to find an unknown angle

Why:

Irregular polygons do not have equal interior angles, so this gives an average value, not the correct unknown angle

Correct move:

Subtract the sum of known interior angles from the total interior sum to find the unknown angle for irregular polygons

Wrong move:

Confusing interior and exterior angle formulas for regular polygons

Why:

Mixing up $\frac{360}{n}$ and $\frac{(n-2)\times 180}{n}$ leads to incorrect values, and you lose method marks for using the wrong formula

Correct move:

Recall that exterior angles always sum to 360°, so calculate the exterior angle first, then subtract from 180° to get the interior angle

Wrong move:

Counting the number of sides of a polygon incorrectly

Why:

Using the wrong n value makes all subsequent calculations wrong, even if your formulas are correct

Correct move:

Count sides twice if not explicitly given, and confirm with the polygon name if provided (e.g. pentagon = 5 sides, hexagon = 6 sides)