How do you divide a circle into eight parts using only three lines?

You can't. However, you can divide it into seven, like so:Make the lines make a triangle in the middle of the circle, rather than making them meet at the centre.

You can cut a round cake into eight parts with three cuts, but you can't cut a circle into eight parts with three straight lines.

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I assume that you want to divide it into three equal "pie slices". If you have the location of the center of the circle, and are allowed to use a protractor (or other angle-measuring device), then...