左=[(1+sinα)/(1+cosα)] *[(1+1/cosα)/(1+1/sinα)]
=[(1+sinα)/(1+cosα)]*[(cosα+1)/cosα]/[(sinα+1)/sinα]
=[(1+sinα)/(1+cosα)]*[(cosα+1)sinα]/[(sinα+1)cosα]
=sinα/cosα
=tanα