lesson
Use a known total to find a missing angle
1 · Learn
Angles on a straight line total 180°, angles around a point total 360° and the interior angles of a triangle total 180°. Vertically opposite angles formed by two crossing straight lines are equal.
Select the fact that matches the diagram and explain why it applies. A drawing may not be to scale, so a measured appearance is not a substitute for stated information.
2 · Worked example
A triangle has angles 48° and 67°. The third is 180 - 48 - 67 = 65°. I can check the three add to 180°.
3 · Your turn
Two adjacent angles form a straight line. One is 124°. Find the other. Then find a triangle's third angle when the first two are 39° and 72°.
Check your answer
56°; 69°.
4 · Apply your learning
Draw and label both situations, then explain the relevant total without relying on the drawing's apparent size.
Adult guidance and safety
Ask the learner to explain the method and check it using a different representation. Supply place-value grids, fraction strips, accurate diagrams and measuring tools where needed. These examples need varied practice; a short correct answer alone does not establish fluency.
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Check a calculation · Year 5 onward
Use this only when your teacher or adult allows a calculator for the task. Show your thinking first. It does not replace mental or written arithmetic and does not mark your lesson answer.
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