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.