What is the geometric property of these 2 angles being equal?

Lebombo

http://www.youtube.com/watch?v=tTnn91KyJ-4

At time 2:20, the woman says, "Using trigonometry, we know that this angle is the same as this angle."

Which trigonometry or geometry property is she referring to that is needed in order to determine those two angles are equal? Is there a name for it? I don't have a geometry book, but I am interested in learning about it. Any information would be helpful.

Thank you.

johnqwertyful

The sum of the angles in a triangle is 180. a+23.5+90=180 a=66.5
Then a+θ=90 66.5+θ=90 θ=23.5

Lebombo

ah, thanks