i.e.,

                                                                           ----------(74)

where is the directional derivative of in the direction of . The equation (74) is called Green's first identity . Interchanging and in the above equation, we get

                             ----------(75)

Subtracting (75) from (74), we get

                  ----------(76)

This is called Green's second identity . Some of the particular cases of these identities give us the following consequences: