■サマーヴィルの等面四面体(その135)

135→(√12,√12,4)に注目して

P1(0,0,0,0)

P3(2,2√2,0,0)

P5(4,0,0,0)

P2(x,y,z,0)とおくと

  x^2+y^2+z^2=7

  (x−2)^2+(y−2√2)^2+z^2=7

  (x−4)^2+y^2+z^2=15

  (x−4)^2+7−x^2=15

  −8x+16=8→x=1

  y^2+z^2=6

  1+(y−2√2)^2+6−y^2=7

  −4√2y+8=0 

y=√2,z=2

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P1(0,0,0,0)

P2(1,√2,2,0)

P3(2,2√2,0,0)

P5(4,0,0,0)

P4(x,y,z,w)とおく.

  P0P1=P1P2=P2P3=P3P4=P4P5=P5P6=P6P7=√7

  P0P2=P1P3=P2P4=P3P5=P4P6=P5P7=√12

  P0P3=P1P4=P2P5=P3P6=P4P7=√15

  P0P4=P1P5=P2P6=P3P7=4

  P0P5=P1P6=P2P7=√15

  P0P6=P1P7=√12

  P0P7=√7

より

  x^2+y^2+z^2+w^2=15

  (x−1)^2+(y−√2)^2+(z−2)^2+w^2=12

  (x−2)^2+(y−2√2)^2+z^2+w^2=7

  (x−4)^2+y^2+z^2+w^2=7

  (x−4)^2+15−x^2=7

  −8x+31=7→x=3

  y^2+z^2+w^2=6

  4+(y−√2)^2+(z−2)^2+w^2=12

  1+(y−2√2)^2+z^2+w^2=7

  1+(y−2√2)^2+6−y^2=7

  −4√2y+8+7=7→y=√2

  z^2+w^2=4

  (z−2)^2+w^2=8

  (z−2)^2+4−z^2=8

  −2z=0→z=0,w=2

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