tanA/2 * tanC/2=(sinA/2*sinC/2)/(cosA/2*cosC/2)
=(sinB/2)/(cos((A+C)/2)+(sinA/2*sinC/2))
=(sinB/2)/(cos((A+C)/2)+(sinB/2))=(sinB/2)/(cos((π-B)/2)+(sinB/2))
=(sinB/2)/(sinB/2+sinB/2)=1/2
tanA/2 * tanC/2=(sinA/2*sinC/2)/(cosA/2*cosC/2)
=(sinB/2)/(cos((A+C)/2)+(sinA/2*sinC/2))
=(sinB/2)/(cos((A+C)/2)+(sinB/2))=(sinB/2)/(cos((π-B)/2)+(sinB/2))
=(sinB/2)/(sinB/2+sinB/2)=1/2