EF is a fixed point. According to symmetry, the perimeter is always equal to EM+MN+NF. If the position of MN is changed casually, EM+MN+NF is a broken line, which is of course longer than a straight line, so the MN with the shortest circumference is the intersection of EF line and two sides.

The knowledge of minimum area junior high school is difficult to understand, and the establishment of coordinate system can be solved by analytic geometry.