tan^2a=2tan^2⊙+1
sin^2a/(1-sin^2a)=2sin^2⊙/(1-sin^2⊙)+1
sin^2a/(1-sin^2a+sin^2a)
=(sin^2⊙+1)/(1-sin^2⊙+sin^2⊙+1)
sin^2a=(sin^2⊙+1)/2
所以：
sin^2⊙=2sin^2a-1