Mandy asked Rob if he could construct a 4 inch circle with a straight
edge and a compass. He said he could. She asked him to do it. Could
he? Why or why not?

Hello teatea!
I'll assume here that a 4-inch circle means that the circle has a
circumference of 4 inches. If this is not the case, please request a
clarification.

If the circumference of a circle is 4 inches, then its diameter must
be 4/Pi inches (Pi is approximately 3.1415...), so it must be 1.273
inches. Now, in order to draw a circle with a compass we need to find
the radius of the circle. All points in a circle are at equal distance
from the center of it, and that distance is the radius. The radius is
simply calculated as half the diameter. Therefore, the radius is
0.636.

Finally. knowing the radius, a compass and a straight edge (and a
ruler) can be used in order to draw the circle. Open the compass 0.636
inches (as measured in the straight edge or ruler) and draw a circle
with it. This circle will have a 4 inch circumference.

You might want to check the following link for more information:

Lessons on Circumference
http://www.mathgoodies.com/lessons/vol2/circumference.html


Google search strategy
circumference circle formula
://www.google.com.ar/search?hl=es&ie=ISO-8859-1&q=circumference+circle+formula&meta=


Best wishes!
elmarto

Hm, it seemed to me the whole point of this problem is to distinguish
"straight-edge" from "ruler." In this case, the circle is not
possible, because he only has a straight-edge. Just my beef.

- Entropix

Even given a ruler, it's an old, old axiom that one cannot draw a line
of length pi, so in geometry (as opposed to engineering which has a
concept of precision) it's impossible to construct a circle with
circumference=4.