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1. Taking a scale or ruler, measure 3.5 cm with the compass.
2. Draw a circle with the compass. mark the centre.
3. Mark a point A on the left side of the circle, on the circumference.
4. Join AO and extend it till it touches the other side of circle at B. 
5. So AB is the diameter.
6. Now take any point C on the circumference. Join AC and BC. Measure the angle with the triangle ruler or protractor.  It will be 90 deg.
7. Take another point D on the circumference on the other side or the same side.  Again join AD and BD. Measure the angle ADB with protractor or triangle ruler.
  it will be 90 deg.