(1) M and n are not in BCD plane, but MN//CD because m and n are the midpoint of AC and AD respectively. Because CD is on BCD plane, MN//BCD plane.

(2) Actually, it is to find the angle ACB, and the triangle ABC is a right triangle, so the length of AC is 2, so the angle ACB=30 degrees (sin (angle ACB)= 1/2).