Given a segment of the length one, construct a segment of the length $\sqrt{2}$ units using a compass only.
Most people start with trying to construct a triangle with the sides 1, 1, $\sqrt {2}$. But this does not work for compass only construction. Instead, it is much easier to construct a triangle $\sqrt{3}$, $\sqrt{3}$, 2, as shown below.
Step by step:
1. Draw a circle of radius 1 with a center at arbitrary point A.
2. Choose arbitrary point B on this circle. Draw a circle of radius 1 with the center at B.
3. The second circle intercepts the first one at points C and D. CD = $\sqrt{3}$
4. Draw the third unit circle with the center at C. It intercepts the first one at B and E. DE = 2.
5. Draw two circles of radius $\sqrt{3}$ with centers at D and E. The distance from any of two points of their intersection to A is equal to $\sqrt{2}$.
The picture was made with Geogebra, and I apologize for its quality due to my first try of this program.
And at the end one more construction problem that involves both traditional instruments - straight edge and compass.
Given three arbitrary parallel lines on a plane, construct an equilateral triangle with vertices on these lines.
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