Prove it — just do the practice exercise.

CMaj7 3rd Inversion

B: 7
C: R
E: 3
G: 5

In 3rd Inversion the 7th is on the bottom:

• 7 – R – 3 – 5

Sounds bright and open. The 7th resolves naturally downward.

Example: CMaj7/B = B + C + E + G