cosαcosα/2cosα/2^2cosα/2^3……cosα/2^n-1
=cosαcosα/2cosα/2^2cosα/2^3……cosα/2^n-1 * (sinα/2^n-1)/ (sinα/2^n-1)
=(1/2)cosαcosα/2cosα/2^2cosα/2^3cosα/2^n-2sinα/2^n-2/ (sinα/2^n-1)
=(1/2)^n-1sin2α/(sinα/2^n-1)