M(-3，0)，F 1(-2，0)

The straight line of MAB is y=k(x+3).

Simultaneous elliptic equation and MAB linear equation eliminate y to get the quadratic equation of x;

Vieta theorem is used to find the sum of two xa+xb and the product of two xa*xb (because it is simplified later), both of which are expressions of k;

It is necessary to prove that BF 1 c * * * line is equivalent to BF1slope =BC slope, which can simplify the proof.