■基本単体の二面角(その184)

 E7では(R/ρ)^2=3

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 αn:aj=(2/j(j+1))^1/2,an=?

 βn:bj=(2/j(j+1))^1/2,bn-1=√2/(n-1),bn=?

 R^2=1+1/3+1/6+1/10+1/15+1/21+a7^2

=1+1/3+1/6+1/10+1/15+2/6+b7^2

 ρ^2=b7^2

 1+1/3+1/6+1/10+1/15=(30+10+5+3+2)/=5/3

 R^2=5/3+1/3+b7^2=5/3+1/21+a7^2

 ρ^2=b7^2

 (2+b8^2)/b7^2=3→b7=1

  1/21+a7^2=1/3+1

  a7^2=27/21=9/7→a7=3/√7

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