A Result of Curve with Tangent and Secant Function as Constant

The question of bitangent circle of closed plane curve is very important to shape regintion about plane area,and the tangent and secant function is also important to the research of bitangent circle.By using the differential geometry technique,extended tangent and secant funcitons of plane curve and space curve are redefined,the situation of the extended tangent and secant function of curve being constant is studied and a necessary and sufficient condition is obtained.At last,a sufficient condition of the extended tangent and secant function of space curve as constant is obtained.