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Mohr–Mascheroni theorem
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=== Outline === To prove the theorem, each of the [[Compass-and-straightedge construction#The basic constructions|basic constructions of compass and straightedge]] need to be proven to be possible by using a compass alone, as these are the foundations of, or elementary steps for, all other constructions. These are: #Creating the line through two existing points #Creating the circle through one point with centre another point #Creating the point which is the intersection of two existing, non-parallel lines #Creating the one or two points in the intersection of a line and a circle (if they intersect) #Creating the one or two points in the intersection of two circles (if they intersect). '''#1 - A line through two points''' It is understood that a straight line cannot be drawn without a straightedge. A line is considered to be given by any two points, as any such pair define a unique line. In keeping with the intent of the theorem which we aim to prove, the actual line need not be drawn but for aesthetic reasons. '''#2 - A circle through one point with defined center''' This can be done with a compass alone. A straightedge is not required for this. '''#5 - Intersection of two circles''' This construction can also be done directly with a compass. '''#3, #4 - The other constructions''' Thus, to prove the theorem, only compass-only constructions for #3 and #4 need to be given.
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