因为：tan(a + b) = ( tana + tanb)/(1 - tana*tanb)所以 tan(a + b)( 1 - tana * tanb) = tana + tanb tan(a + b) - tana * tanb * tan(a + b) = tana + tanb即： tana + tanb = tan(a +b) - tana * tanb * tan(a + b)
