Point m is the symmetrical point of point p with respect to A0.

So according to the nature of symmetry, there are:

PE = me

Similarly, pf = nf

So Mn = me+ef+nf

=PE+EF+PF

= perimeter of triangle PEF

= 15

For reference! I wish you progress in your studies.

The original ID "Jiangsu Wuyunchao" is known to be unusable in Baidu, and it has been permanently sealed (the number of nearly 30,000 is actually not cheating). I suggest that you don't cheat, don't score points, and operate in a standardized way. Otherwise, it's no use complaining after turning it off.