Pass any point p on the intersection line l of a and b (p does not belong to c)
Let m be the vertical line of c, because m is perpendicular to c and plane a.
Perpendicular to c, p belongs to a, and a straight line m can be obtained.
In the same way, it can be concluded that m also belongs to B. place
Therefore, m is the intersection of a and B.
Complete the certificate.