Obviously, the normal vector of the plane ADD 1A 1 is the y axis {0, 1, 0}, as long as it is proved that MN is perpendicular to the y axis;

According to the set coordinates and the length of each edge line of the cuboid, the coordinates of relevant points are as follows:

D(0，0，0)，A(a，0，0)，B(a，2a，0)，C(0，2a，0)，D 1(0，0，A)，M(3a/4，A，0)，N(。

So the vector Mn = {-3a/4,0, a/2};

Because {0, 1, 0}? {-3a/4，0，a/2 } = 0 *(-3a/4)+ 1 * 0+0 *(a/2)= 0；

So the vector MN is perpendicular to the Y axis, so Mn ∑ add1a1;