t+T

�c = �c t dt
+"
T
t



t+T


�c = �� s �t - kx +� dt = �� s
+"
T
t

� t+T -kx+�
t+T

�t - kx +� dt = z dz =
+" +"
T � T
t �t-kx+�

� t+T -kx+�

= - z dz =
[ ]
+"
� T
�t-kx+�
z = �t - kx + � dz = � dt