Take three hands-up answers before confirming half. Do not say the area rule yet, let the word "half" hang so it pays off in Watch and Notice.

Right-angled, base 6, height 4: box overlay shows the triangle is half of the 6×4 rectangle, whole box 24 cm², so ½ × 6 × 4 = 12 cm². Point at the two equal halves.

Scalene, base 8, height 5: ask the class to predict first, then ½ × 8 × 5 = 20 cm².

Isosceles, base 10, height 6: ½ × 10 × 6 = 30 cm².

Obtuse, base 9, height 4: perpendicular height drops outside the base. Hold out for this — the rule never flinches, ½ × 9 × 4 = 18 cm². Point to the slanting side and ask "is this the height?" — no, and that is the whole lesson.

This is the guided practice round, we talk it through together, pupils take turns at the board and the class agrees or corrects out loud.

Drag the triangle legs in explore mode; the area readout confirms each prediction. The drag only needs to be close, the ½ × b × h calculation is the source of truth and the live readout confirms it. Sketch the enclosing rectangle by hand each time (the explore interactive shows one triangle at a time) so the half-rectangle picture is always on view.

• base 5, height 4 → ½ × 5 × 4 = 10 cm²
• base 5, height 8 → 20 cm² (same base, taller, the height doubled, so the area doubled)
• base 12, height 3 → 18 cm² (wide and low, pupils often expect it to be the biggest; it isn't)

Ask "why did doubling the height double the area, but the wide flat one is smaller?", hold out for pupils naming that both base and height decide the area.

This is the practice round, pupils take turns at the board, check each answer, and the class confirms before moving on.

Keep the board work brisk rather than over-explaining.

• base 4, height 6 → 12 cm²
• base 10, height 5 → 25 cm²
• base 7, height 4 → 14 cm²
• base 6, height 6 → 18 cm²

The four straight ones build from small to a square-ish case to the flat one, pupils get faster each time. Then the find-the-height stretch reverses the rule: area 18 = ½ × 9 × h, so 18 = 4.5 × h, so h = 4 cm. Work this one on the board without the interactive, the shape-measurer only reads area off a dragged shape, so it cannot check a missing height. Work back from the area rather than forward, this is where the strongest pupils stretch and the rest see the rule run in reverse.