Trigonometry is a branch of mathematics that deals with the relationships between the sides and angles of triangles. The three terms in this crossword are all related to trigonometry. COSINE (6 letters) is a trigonometric function that is equal to the ratio of the adjacent side to the hypotenuse of a right triangle. SINE (4 letters) is another trigonometric function that is equal to the ratio of the opposite side to the hypotenuse of a right triangle. SECANT (6 letters) is a trigonometric function that is equal to the ratio of the difference between two sides of a triangle to the sum of those two sides.