Maybe, Maybe not

Geometry Level 3

Can a straight line be divided in the ratio ( 2 3 ) : ( 2 + 3 ) (2 - \sqrt{3}) : (2 + \sqrt{3}) by using only compass and straightedge?

Depends on the length of line No Yes

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2 solutions

Ivan Koswara
Jan 20, 2017

Since ( 2 3 ) : ( 2 + 3 ) = 1 : ( 7 + 4 3 ) (2 - \sqrt{3}) : (2 + \sqrt{3}) = 1 : (7 + 4\sqrt{3}) , we're looking to construct 1 1 + ( 7 + 4 3 ) = 1 8 + 4 3 = 1 4 ( 2 3 ) \frac{1}{1 + (7 + 4\sqrt{3})} = \frac{1}{8 + 4\sqrt{3}} = \frac{1}{4} (2 - \sqrt{3}) of a line segment. Since this is a constructible number, the answer is yes .

One example construction: Call our line segment A B AB .

  1. Construct the midpoint of A B AB , call it C C . (Standard construction: circle with center A A and radius A B AB and circle with center B B and radius B A BA intersect at two points X , Y X, Y ; X Y XY and A B AB intersect at the desired C C .)
  2. Construct the midpoint of A C AC , call it D D . (Same construction.)
  3. Draw a perpendicular of D A DA at D D , call it l 1 l_1 . (Standard construction: circle with center D D and radius D A DA intersects A D AD at a point Z Z (which happens to be C C , but we'll generalize); now construct midpoint of A Z AZ with the above standard construction, we will get the line X Y XY that intersects A Z AZ at D D . This time, we take X Y XY instead; it's the perpendicular.)
  4. Circle with center D D and radius D A DA intersects l 1 l_1 at two points E , E E, E' .
  5. Circle with center C C and radius C E CE intersects the interior of A C AC at a point F F .
  6. Draw a perpendicular of F A FA at F F , call it l 2 l_2 .
  7. Draw a perpendicular of E D ED and E E , call it l 3 l_3 .
  8. l 2 l_2 and l 3 l_3 intersect at G G .
  9. Circle with center C C and radius C G CG intersects the interior of A C AC at a point H H .
  10. Now A H : H B = ( 2 3 ) : ( 2 + 3 ) AH : HB = (2 - \sqrt{3}) : (2 + \sqrt{3}) .

Proving it is easy. Treat A B AB as having 4 units length. Then A D = D C = 1 4 A B = 1 AD = DC = \frac{1}{4} AB = 1 . Also, D E = D A = 1 DE = DA = 1 , so C E = C F = 2 CE = CF = \sqrt{2} . Since A B AB is perpendicular to both D E DE and F G FG , the latter two are parallel. Since A B AB is parallel to E G EG , D E G F DEGF is a rectangle, so F G = D E = 1 FG = DE = 1 . Thus C G = C H = 3 CG = CH = \sqrt{3} . So A H = 2 3 AH = 2 - \sqrt{3} , and by subtraction, H B = A B A H = 2 + 3 HB = AB - AH = 2 + \sqrt{3} .

Oh! you have got no diagrams in a construction.

Vishwash Kumar ΓΞΩ - 4 years, 4 months ago

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Yes; I started drawing with GeoGebra but soon found the resulting drawing too messy. And I'm too lazy to make several diagrams.

Ivan Koswara - 4 years, 4 months ago

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