■等面単体の体積(その247)

  P0P1=P1P2=P2P3=P3P4=2

  P0P2=P1P3=P2P4=√6

  P0P3=P1P4=√6

  P0P4=2

  P1(0,0,0,0)

  P2(2,0,0,0)

  P3(3/2,(√5)/2,(√10)/2,0)

  P4(1,√5,0,0)

  P0(x,y,z,w)

とおくと,

  x^2+y^2+z^2+w^2=4

  (x−2)^2+y^2+z^2+w^2=6

  (x−3/2)^2+(y−(√5)/2)^2+(z−(√10)/2)^2+w^2=6

  (x−1)^2+(y−√5)^2+z^2+w^2=4

  (x−2)^2+4−x^2=6

  −4x+4+4=6,x=1/2

  y^2+z^2+w^2=4−1/4=15/4

  1/4+(y−√5)^2+15/4−y^2=4

  −2√5y+5=0,y=(√5)/2

  z^2+w^2=10/4

  1+0+(z−(√10)/2)^2+10/4−z^2=6 

  1−(√10)z+10/4+10/4=6,z=0,(√10)/2

  P0(1/2,(√5)/2,0,(√10)/2)

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