[Keith Vetter] 2003-02-07 - a simple whizzlet visualizing Morley's Miracle theorem. Just drag any vertex and watch it work.

About a century ago, Frank Morley proved a curious theorem about triangles which has become known in mathematical folklore as Morley's Miracle.

The theorem states that: "The three points of intersection of the adjacent trisectors of the angles of any triangle form an equilateral triangle."

See http://www.cut-the-knot.com/triangle/Morley/Morley.shtml for more details.

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 #!/bin/sh -*- tab-width: 8; -*-
 # The next line is executed by /bin/sh, but not tcl \
 exec wish $0 ${1+"$@"}
 ##+##########################################################################
 #
 # morley.tcl -- a whizzlet visualizing Morley's Miracle 
 # see http://www.cut-the-knot.com/triangle/Morley/Morley.shtml
 # by Keith Vetter
 #
 # Revisions:
 # KPV Feb 07, 2003 - initial revision
 #
 #############################################################################
 package require Tk
 array set P {1 {200 50} 2 {76 381} 3 {472 309}} ;# Initial position
 
 proc DoDisplay {} {
    global P
 
    wm title . "Morley's Miracle"
    canvas .c -width 500 -height 500 -bd 2 -relief raised
    pack .c -side top -fill both -expand 1
    button .about -text About -command About
    .c create window 5 5 -window .about -anchor nw -tag about
 
    foreach w {1 2 3} {
        .c create oval [Box $P($w)] -tag [list vert p$w] -fill red
        .c bind p$w <B1-Motion> [list DoButton $w %x %y]
    }
    bind all <Alt-c> [list console show]
 }
 ##+##########################################################################
 # 
 # DoButton -- interactively moves a vertex around and redraws everything
 # 
 proc DoButton {who X Y} {
    if {$X > [winfo width .c] - 5 || $Y > [winfo height .c] - 5} return
    foreach {x y} $::P($who) break
    set ::P($who) [list $X $Y]
    .c move p$who [expr {$X - $x}] [expr {$Y - $y}]
    DrawLines
 }
 ##+##########################################################################
 # 
 # Box -- returns coordinates for a box around a given point
 # 
 proc Box {xy} {
    foreach {x y} $xy break
    return [list [expr {$x-5}] [expr {$y-5}] [expr {$x+5}] [expr {$y+5}]]
 }
 ##+##########################################################################
 # 
 # VAdd -- adds two vectors w/ scaling of 2nd vector
 # 
 proc VAdd {v1 v2 {scaling 1}} {
    foreach {x1 y1} $v1 {x2 y2} $v2 break
    return [list [expr {$x1 + $scaling*$x2}] [expr {$y1 + $scaling*$y2}]]
 }
 ##+##########################################################################
 # 
 # VSub -- subtract two vectors
 # 
 proc VSub {v1 v2} { return [VAdd $v1 $v2 -1] }
 ##+##########################################################################
 # 
 # DrawLines -- draws all the various lines on the screen
 # 
 proc DrawLines {} {
    global P
    .c delete outer tri equi
    
    .c create line [concat $P(1) $P(2) $P(3) $P(1)] -tag outer -width 2
    DrawTrisectors
    .c raise vert
 }
 ##+##########################################################################
 # 
 # DrawTrisectors -- draws the angle trisectors out to where they
 # meet and then draws the Morley triangle in the middle.
 # 
 proc DrawTrisectors {} {
    global P
 
    # Get trisector lines out of each vertex
    foreach {t(1,1) t(1,2)} [TrisectAngle $P(3) $P(1) $P(2)] break
    foreach {t(2,1) t(2,2)} [TrisectAngle $P(1) $P(2) $P(3)] break
    foreach {t(3,1) t(3,2)} [TrisectAngle $P(2) $P(3) $P(1)] break
 
    # Find where trisector line segments intersect
    set E1 [Intersect $P(1) $t(1,1) $P(2) $t(2,2)]
    set E2 [Intersect $P(2) $t(2,1) $P(3) $t(3,2)]
    set E3 [Intersect $P(1) $t(1,2) $P(3) $t(3,1)]
    if {$E1 == {} || $E2 == {} || $E3 == {}} return ;# Colinear lines?
    
    .c create line [concat $P(1) $E1 $P(2) $E2 $P(3) $E3 $P(1)] -tag tri \
        -fill blue
    .c create line [concat $E1 $E2 $E3 $E1] -tag equi -fill red -width 2
 }
 ##+##########################################################################
 # 
 # Intersect -- find where two line intersect given two points on each line
 # 
 proc Intersect {p1 p2 p3 p4} {
    foreach {x1 y1} $p1 {x2 y2} $p2 {x3 y3} $p3 {x4 y4} $p4 break
 
    set numer [expr {($x4 - $x3)*($y1 - $y3) - ($y4 - $y3)*($x1 - $x3)}]
    set denom [expr {($y4 - $y3)*($x2 - $x1) - ($x4 - $x3)*($y2 - $y1)}]
    if {$denom == 0} return
 
    set X [expr {$x1 + ($numer / $denom) * ($x2 - $x1)}]
    set Y [expr {$y1 + ($numer / $denom) * ($y2 - $y1)}]
    return [list $X $Y]
 }
 ##+##########################################################################
 # 
 # TrisectAngle -- returns two points which are on the two lines trisecting
 # the angle created by points p1,p2,p3. We use the cross product to tell
 # us clockwise ordering.
 # 
 proc TrisectAngle {p1 p2 p3} {
    set cross [Cross [VSub $p2 $p1] [VSub $p2 $p3]]
    if {$cross < 0} {foreach {p1 p3} [list $p3 $p1] break}
    
    set theta [FindAngle3 $p1 $p2 $p3]          ;# What the angle is
    set theta1 [expr {$theta / 3.0}]            ;# 1/3 of that angle
    set theta2 [expr {2 * $theta1}]             ;# 2/3 of that angle
 
    set v [VSub $p3 $p2]                        ;# We'll rotate this leg
    set v1 [RotateCC $v $theta1]                ;# By 1/3
    set v2 [RotateCC $v $theta2]                ;# By 2/3
    set t1 [VAdd $p2 $v1]                       ;# Trisect point 1
    set t2 [VAdd $p2 $v2]                       ;# Trisect point 2
 
    if {$cross < 0} { foreach {t1 t2} [list $t2 $t1] break }
    return [list $t1 $t2]
 }
 ##+##########################################################################
 # 
 # FindAngle3 -- returns the angle between three points
 # 
 proc FindAngle3 {p1 p2 p3} {
    foreach {x1 y1} [VSub $p1 $p2] {x2 y2} [VSub $p3 $p2] break
    
    set m1 [expr {sqrt($x1*$x1 + $y1*$y1)}]
    set m2 [expr {sqrt($x2*$x2 + $y2*$y2)}]
    set dot [expr {$x1 * $x2 + $y1 * $y2}]
 
    set theta [expr {acos($dot / $m1 / $m2)}]
    return $theta
 }
 ##+##########################################################################
 # 
 # RotateCC -- rotates vector v by beta radians counter-clockwise
 # 
 proc RotateCC {v beta} {
    foreach {x y} $v break
    set xx [expr {$x * cos(-$beta) - $y * sin(-$beta)}]
    set yy [expr {$x * sin(-$beta) + $y * cos(-$beta)}]
    return [list $xx $yy]
 }
 ##+##########################################################################
 # 
 # Cross -- returns the cross product -- easy w/ z=0
 # 
 proc Cross {v1 v2} {
    foreach {x1 y1} $v1 {x2 y2} $v2 break
    return [expr {($x1*$y2) - ($y1*$x2)}]
 }
 proc About {} {
    set msg "Morley's Miracle\nby Keith Vetter, Feb 2003\n\n"
    append msg "A whizzlet for visualizing Morley's Miracle. Drag any\n"
    append msg "vertex and see Morley's Miracle in action.\n\n"
    append msg "About a century ago, Frank Morley proved a curious\n"
    append msg "theorem about triangles which has become known in\n"
    append msg "mathematical folklore as Morley's Miracle.\n\n"
    append msg "The theorem states that:\n"
    append msg "    The three points of intersection of the adjacent \n"
    append msg "    trisectors of the angles of any triangle form an \n"
    append msg "    equilateral triangle.\n\n"
    append msg "See http://www.cut-the-knot.com/triangle/Morley/Morley.shtml\n"
    append msg "for more details."
    tk_messageBox -title "About Morley's Miracle" -message $msg
 }
   
 ################################################################
 DoDisplay
 DrawLines
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Wouldn't it be nice it the vertices of the outer triangle could be
dragged around?
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[Category Graphics] | [Category Mathematics] | [Category Application] | [Category Whizzlet]