The solution to this tough nut was submitted by Ray who does not reveal his
school. By Pythagoras Theorem:
As
it follows, by the converse of
Pythagoras Theorem, that
.
Using the cosine rule:
So
and
degrees to the nearest degree.
The challenge was to find more than one method so the second method uses
geometry and vectors.
Triangle
is a right angled isosceles triangle
and
is perpendicular to
.The line
is perpendicular to the plane
so it is
perpendicular to
. As the two line
and
are perpendicular to
the whole plane
is
perpendicular to
. Hence
is perpendicular to
.