You will construct an isosceles triangle ABC with an angle of 120° at B. You will do this by constructing the two base angles. If you remember your triangle facts, then it should be clear to you that both base angles must be 30°.

Keep in mind that in your construction, you can only use the black objects. The red objects are what you will obtain when you finish the construction.

• Choose the 'angle with fixed size' tool .
•  Click on A, C then somewhere close to B. In the dialog box, enter 30 for the size of the angle. In order to show the angle's size, click the 'show object value' icon . Then click 'OK'.
• Then click on C, A and close to B. In the dialog box, enter 30 again. 
• Create B with the point tool, at the intersection of the two rays. "Well done!" appears.

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