If the angle between vector a = (3, m, 4) and B = (- 2, 2, m) is obtuse, then the value range of M, please write the detailed process, the answer is m

If the angle between vector a = (3, m, 4) and B = (- 2, 2, m) is obtuse, then the value range of M, please write the detailed process, the answer is m

Cos angle = (vector a hits vector b) / (module of vector a multiplies module of vector b)