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divide a line segment into three equal parts

Amer

Active member
Mar 1, 2012
275
is there a way to divide a line segment into three equal parts using just compass and ruler ?
I heard that there is not a way and there is a proof for that is that right ?
 

caffeinemachine

Well-known member
MHB Math Scholar
Mar 10, 2012
834
Re: divde a line segment into three equal parts

is there a way to divide a line segment into three equal parts using just compass and ruler ?
I heard that there is not a way and there is a proof for that is that right ?
It is possible. Draw a line segment AB. draw a ray beginning at A at an angle of 60 degrees to AB. draw another ray beginning at B at an angle of 60 degrees from BA(Note that the second ray is in opposite direction of first ray). Take any arbitrary distance on the compass. Put pointy end of compass on point A and cut first ray at point K. put pointy end of compass on K and cut first ray again to get point L. Put pointy end of compass on B and cut second ray to get point M and now put pointy end of compass on M and cut second ray to get point N. join KN and LM. The line segment AB is now trisected.
 

Ackbach

Indicium Physicus
Staff member
Jan 26, 2012
4,192
Re: divde a line segment into three equal parts

Perhaps you're thinking of an angle trisection instead of a line trisection? It is true that the angle cannot, so far as we know, be trisected using straightedge and compass.
 

caffeinemachine

Well-known member
MHB Math Scholar
Mar 10, 2012
834
Re: divde a line segment into three equal parts

is there a way to divide a line segment into three equal parts using just compass and ruler ?
I heard that there is not a way and there is a proof for that is that right ?
Angles, in general, cannot be trisected using (unmarked)straight edge and compass alone. In "Abstract Algebra" texts, example Herstein's "Topics in Algebra", it is proved that and 60 degree cannot be trisected using unmarked straight edge and compass alone.
 

Amer

Active member
Mar 1, 2012
275
Re: divde a line segment into three equal parts

how to draw an angle of 60 ? with straight ruler unmarked with compass
that cant be done
 

Plato

Well-known member
MHB Math Helper
Jan 27, 2012
196
Re: divde a line segment into three equal parts

is there a way to divide a line segment into three equal parts using just compass and ruler ?
I heard that there is not a way and there is a proof for that is that right ?
We can divide any line segment into any finite natural number of congruent sub-segments.
Start with $\overline {AB} $. At $A$ draw any ray not collinear with $\overrightarrow {AB}$.
Now on that ray starting at $A$ mark off three points $E,~F,~\&~G$ so that $\overline {AE},~\overline {EF},~\&~\overline {FG}$ have the same length.
Join $G~\&~B$ with a line. Construct at $E~\&~F$ lines parallel $\overline {GB}$.
Those lines will trisect $\overline {AB}$.
 

chisigma

Well-known member
Feb 13, 2012
1,704
Re: divde a line segment into three equal parts

The segment AB that must divide into three parts is represented in the figure...

MHB11.PNG

The following procedure requires only a non graduaded rule and a compass...

a) construct the equilateral triangle ABC...

b) construct the segments DA=AB and BE=AB collinear to AB...

c) draw the segments DC and CE...

d) draw vertical lines passing through A and B a call F and I the intersection point with the segments DC and CE...

e) draw the horizontal segment FI and call G and H the intersection points with the segments AC and BC...

At this point we have the segment FI that is equal to AB and is divided into three equal segments FG, GH and HI...

Kind regards

$\chi$ $\sigma$
 

earboth

Active member
Jan 30, 2012
74
Re: divde a line segment into three equal parts

how to draw an angle of 60°? with straight ruler unmarked with compass
that cant be done
Draw an equilateral triangle with unmarked straight ruler and compass. Then you even have got three angles with 60°.
 

Amer

Active member
Mar 1, 2012
275
Re: divde a line segment into three equal parts

thank you all,thats great :D
 

HallsofIvy

Well-known member
MHB Math Helper
Jan 29, 2012
1,151
Re: divde a line segment into three equal parts

how to draw an angle of 60 ? with straight ruler unmarked with compass
that cant be done
It surely can be done. Use the ruler to draw a straight line. Mark any two points on the line and call them "A" and "B". Using the compass strike a circle through "B" having center "A". Using the compass strike a circle through "A" with center "B". Those circles with intersect in two points. Choose either of them and call it "C". The angle CAB will have measure 60 degrees.