The theorem stays valid when Z 1 {\displaystyle Z_{1}} coincides with P {\displaystyle P} or Q {\displaystyle Q} , provided one replaces the then undefined line Z 1 P {\displaystyle Z_{1}P} or Z 1 Q {\displaystyle Z_{1}Q} by the tangent on k 1 {\displaystyle k_{1}} at Z 1 {\displaystyle Z_{1}} .