# Trigonometry - Cosine of angle

posted by
**Knights**
.

What is the cosine of the angle between two adjacent faces of a regular tetrahedron? (We define the angle between two intersecting planes as the angle between two intersecting lines, one in each plane, such that each line is perpendicular to the line at which the planes intersect.)

I don't understand....it is not cosine 60, so could someone help?