左=sinA/cosA + cos(π/4+A/2) /sin(π/4+A/2)
=sinA/cosA + [cosπ/4 cosA/2-sinπ/4 sinA/2]/[sinπ/4 cosA/2+sinA/2cosπ/4]
=sinA/cosA + [cosA/2- sinA/2]/[ cosA/2+sinA/2]
=sinA/cosA + [cosA/2- sinA/2][cosA/2- sinA/2]/[ cosA/2+sinA/2][cosA/2- sinA/2]
=sinA/cosA + [cos^2 A/2- 2sinA/2cosA/2-+sin^2 A/2]/cos^2 A/2- sin^2 A/2]
=sinA/cosA +(1-sinA)/cosA
=1/cosA =右边