|\
tanasinn→ |∴\←?
|∴∵\
~ ̄ ̄ ̄
↑
tanasinn

|\
tanasinn→ |∴\←tanisige
|∴∵\
~ ̄ ̄ ̄
↑
tanasinn

A
|\
α→ |∴\ ∠ABC=θ
|∴∵\
B~ ̄ ̄ ̄ C
↑
β

S = 1/2・α・β・tanasinθ= tanasin.