帮忙讲解下这道题目---7. (2016 秋•安图县期末) 如图，在△ABC 中，AB = AC，AB 的垂直平分线交 AB 于 M，交 AC 于 N. (1) 若∠ABC = 75°，则∠MNA 的度数是 (2) 连接 NB，若 AB = 10cm，△NBC 的周长是 18cm . ①求 BC 的长； ②在直线 MN 上是否存在 P，使由 P、B、C 构成的△PBC 的周长值最小？若存在，标出点 P 的位置并求△PBC 的周长最小值；若不存在，说明理由 Chart/Diagram Description: * **Type:** Geometric figure (Triangle with a line). * **Main Elements:** * **Shape:** A triangle labeled ABC. * **Points:** A, B, C are vertices of the triangle. M is a point on the line segment AB. N is a point on the line segment AC. * **Lines:** * Line segments AB, BC, AC form the triangle. * A straight line passes through M and N. This line is depicted intersecting AB at M and AC at N. Based on the problem description, this line is the perpendicular bisector of AB and intersects AB at M and AC at N. * A dashed line segment NB connects points N and B. * **Labels:** Points A, B, C, M, N are labeled.