= pa2+( 1-p)B2-p( 1-p)C2

=b2+pa2-pb2-pc2+p2c2

=b2+p(a2-b2-c2)+p2c2

=b2+p(-2bc*cosA)+p2c2

The quadratic function of p, c2>0, is opening.

Discriminant = 4b2cos2a-4b2c2 = 4b2c2 (cos2a-1)

- 1 & lt； cosA & lt 1

Therefore, 4bc2 (cos2a- 1)

The quadratic function does not intersect with the X axis, so this formula is always positive.