Take three hands-up answers, not open call-outs. Listen for pupils separating construct (draw with compass/ruler) from work out (deduce an angle) — that split is the whole lesson.

Talk over the build; do not read a script. Name the three sides (6, 5, 4 cm) and stress that only one triangle closes from them.

Land the key idea on the board where pupils can see it: once two angles are known, write 180° − angle 1 − angle 2 and show the third angle appearing from the subtraction, not from the protractor. Let a pupil say the third value before you complete it.

Watching the board build IS the participation for the class — every pupil follows on their own paper while one pupil works the board build. You do not need every pupil to take a board turn.

Pacing levers across the beat: before the protractor confirms the third angle, ask the class to predict it from 180° minus the other two; turn-and-name a pupil for the subtraction; revoice a strong answer (so we already knew it before we measured). Watch for pupils reading the wrong protractor scale on the second angle.

This runs as a paper/desk investigation — pupils sketch and construct in their copies while you circulate. Hold one completed exemplar design on the board so pupils can see what a finished piece looks like before they start.

The core every pupil must produce is one constructed triangle with one reasoned angle. The circle-with-labelled-chord and the net are optional add-ons for pupils who covered them in earlier lessons — do not require them. Take one or two designs to the board at the end for the class to check the reasoned angle.