Area & perimeter (2D shapes, circles)

Applying one formula to the whole figure instead of splitting it into a rectangle plus a trapezium (etc.).

Finding each piece correctly but omitting one when combining (e.g. not adding a height back on).

Using $2\pi r$ when area $\pi r^2$ is needed (or vice versa).

Putting the diameter $d$ into $\pi r^2$ or $\tfrac{\theta}{360}\pi r^2$ instead of the radius $r=\tfrac{d}{2}$. A diameter of $10$ gives $r=5$, so area $=\pi\times 5^2$.

Writing a triangle area as $bh$ or a trapezium as $(a+b)h$, forgetting the $\tfrac12$. Triangle $=\tfrac12 bh$; trapezium $=\tfrac12(a+b)h$.