sinβ/sinα=cosαcosβ-sinαsinβ
tanβ=sinαcosα-(sinα)^2*tanβ
tanβ=sinαcosα/(1+(sinα)^2)
=sinαcosα/((cosα)^2+2(sinα)^2)
=1/(2sinα/cosα+cosα/sinα)
sinβ/sinα=cosαcosβ-sinαsinβ
tanβ=sinαcosα-(sinα)^2*tanβ
tanβ=sinαcosα/(1+(sinα)^2)
=sinαcosα/((cosα)^2+2(sinα)^2)
=1/(2sinα/cosα+cosα/sinα)