Leo+DadMade for Leo
Area of Trapeziums, Rhombuses & Kites
Rung 3 of 4 · The traps

Using the Wrong Measurement

The formula is the easy part. The marks are lost choosing the wrong line to measure — the slant instead of the height, the side instead of the diagonal.


ExploreTap the lines you'd actually use, then “check my picks”. Watch the decoys.
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Every one of these shapes hands you more numbers than you need. The question is never "can you multiply?" — it's "did you pick the right lines?"

Trapezium: Height, Not Slant

The h in ½(a + b)h is the perpendicular height — the straight-up gap between the two parallel sides, measured at a right angle. It is not the slanted side, even though the slant is right there looking like an obvious length to grab. The slant is always longer than the true height, so using it inflates your answer. In the toy, the slant is offered as a decoy — leave it alone.

Say it plainly: for a trapezium use the two parallel sides and the straight-up height. The slanted edge is a trap, never an input.

Kite & Rhombus: Diagonals, Not Sides

For ½ × d₁ × d₂ you need the two diagonals — the lines that join opposite corners and cross in the middle. The side lengths are not what this formula wants, even though they're the first thing you'd measure with a ruler. A rhombus can have four equal sides of 5 and still have completely different diagonals; the sides alone won't give you the area. Tap the diagonals; ignore the sides.

How to Spot Which Is Which

Two quick checks. For a trapezium, ask: "does this line go straight across the gap at a right angle?" If yes, that's your height. For a kite or rhombus, ask: "does this line run corner-to-corner through the middle?" If yes, that's a diagonal. Get into the habit of pointing at the line before you write any number down — that pause is what saves the marks.

Us, Thinking Out Loud

Why is the slant side always longer than the true height?

Could two kites have the same sides but different areas? How?