When we find missing angles we usually need to communicate how we found them. How can we do this?

You might like to start by revising the angle sum of a triangle and the angle sum of a quadrilateral.

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Open this interactive page and find the value of angle BAC.

The check boxes allow you to reveal or hide the mathematical working. Reveal the working. Is your answer correct? Did you follow the same steps when you worked out the answer?

Find a partner and choose a Player A and a Player B. Follow these steps:

  • Player A hides line 5 and specifies any acute angle to player B, e.g. 60 degrees.

  • Player B adjusts the triangle so that angle BAC is equal to the angle specified by Player A.

  • Player A then reveals line 5 to check if Player B is correct

  • Swap roles with your partner

Play a few rounds, hiding lines 3 and 4 as well as line 5. Then play the same game with quadrilaterals.

Revise the angle sum of a straight line then follow these steps (DOCX 580kB) to create your own GeoGebra applet that shows the mathematical working for finding an angle on a straight line.