Thinking outside of the box

953 Views Asked by At

You want to draw a circle with a 4 inch radius. A trivial task for you and your trusty compass. When you go to grab your compass which has not had much love for a while you find it is rusted shut; stuck at 5 inches. Is it still possible to complete the task and draw a perfect circle with a 4-inch radius using the compass that can only draw circles with a 5-inch radius?

You may use other things as well to solve the problem such as a straightedge.

1

There are 1 best solutions below

6
On BEST ANSWER

My first approach

Yes, just place the center 3 inches above the paper. If that is a possibility?

Different rendering

To put it differently, if you like, you could draw a 5-inch-circle, use scissors to cut a radius, then form a cone by overlapping at the cutting line until the proportion of the height to the radius of the base is 3 to 4...

That will happen when the overlap covers $\frac{1}{4}$ of the surface of the cone...

Illustrations

To fully earn your votes, here is an illustration:

enter image description here

To see the situation from arbitrary angles, consult the following dynamic 3D-graph:

GeoGebra-illustration

In particular, see what it looks like from above - the $1:4$ proportion of the overlap becomes evident!