Contstructing Segments

**ReneePatt** · December 9th 2009, 05:05 AM

Given three segments whose lengths are 1, a, and b, construct segments of length a + b, |a – b|, ab, $\frac{a}{b}$ , and $\sqrt{ab}$ . These are Euclidean constructions.

**Soroban** · December 9th 2009, 05:53 AM

Hello, ReneePatt!

Surely, you can do the first two . . .

Given three segments whose lengths are: $1,\;a,\text{ and }b,$
construct segments of lengths:

$(1)\;a + b \qquad (2)\;|a - b| \qquad (3)\;ab \qquad (4)\;\frac{a}{b} \qquad (5)\;\sqrt{ab}$

$(3)\;ab$

On a horizontal line, measure off: . $OP = 1,\;PQ = a$

Code:



      o - - - o - - - o - - -
      O   1   P   a   Q

Through $O$ draw a diagonal line.
On the diagonal, measure off $OR = b$
Draw $PR.$

Code:

                              *
                           *
                        *
                  R  *
                  o 
           b   * /
            *   /
         *     /
      o - - - o - - - o - - -
      O   1   P   a   Q

Through $Q$ construct a line parallel to $PR$ , cutting the diagonal at $S.$

Code:

                              S
                              o
                       x   * /
                        *   /
                  R  *     /
                  o       /
           b   * /       /
            *   /       /
         *     /       /
      o - - - o - - - o - - -
      O   1   P   a   Q

Then: . $x \,=\,RS \,=\,ab$

Proof
From similar triangles: . $\frac{b+x}{1+a} \:=\:\frac{b}{1}$

And we have: . $b + x \:=\:b + ab \quad\Rightarrow\quad x \:=\:ab$

**earboth** · December 9th 2009, 07:51 AM

Originally Posted by ReneePatt

Given three segments whose lengths are 1, a, and b, construct segments of length ... $\sqrt{ab}$ . These are Euclidean constructions.

1. Draw a line with the length (a + b)

2. This line is the diameter of a circle.

3. Construct a perpendicular line at the end of a = begin of b. This line intersect the circle line. Construct a right triangle with (a + b) as hypotenuse .

4. According to Euclid's theorem you have in a right triangle:

$a \cdot b = h^2~\implies~\boxed{h = \sqrt{a \cdot b}}$

**Soroban** · December 9th 2009, 10:11 AM

Hello, Renee!

$(4)\;\frac{a}{b}$

On a horizontal line, measure off: . $OP = b,\;PQ = 1$

Code:



      o - - - o - - - o - - -
      O   b   P   1   Q

Through $O$ draw a diagonal line.
On the diagonal, measure off $OR = a$
Draw $PR.$

Code:

                              *
                           *
                        *
                  R  *
                  o 
           a   * /
            *   /
         *     /
      o - - - o - - - o - - -
      O   b   P   1   Q

Through $Q$ construct a line parallel to $PR$ , cutting the diagonal at $S.$

Code:

                              S
                              o
                       x   * /
                        *   /
                  R  *     /
                  o       /
           a   * /       /
            *   /       /
         *     /       /
      o - - - o - - - o - - -
      O   b   P   1   Q

Then: . $x \,=\,RS \,=\,\frac{a}{b}$

Proof

From similar triangles: . $\frac{a+x}{b+1} \:=\:\frac{a}{b}$

And we have: . $ab+bx \:=\:ab +a \quad\Rightarrow\quad bx \:=\:a \quad\Rightarrow\quad x \:=\:\frac{a}{b}$

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