>556

|a[0]| ≦1 のとき
　a[k] = cos(θ/(2^k)) = sin(θ/(2^(k-1)))/2sin(θ/(2^k)),　
　θ = ±arccos(a[0]),
　a[1]a[2]…a[n] = sinθ/{(2^n)sin(θ/(2^n))} → (sinθ)/θ.

|a[0]|≧1 のとき
　a[k] = cosh(t/(2^k)) = sinh(t/(2^(k-1)))/2sinh(t/(2^k)),
　t = ±log(a[0]+√{a[0]^2 -1}).
　a[1]a[2]…a[n] = sinh(t)/{(2^n)sinh(t/(2^n))} → sinh(t)/t.