1x 21/23=21/23 i need help with this question?

Express the following as a function of a single angle, cos(60) cos(-20) - sin(60) sin(-20)

Apublic library wants to place 4 magazines and 9 books on each display shelf. the expression 4s+9s represents the total number of items that will be displayed on s shelves. simplify the expression

What is the relationship between all triangles that can be drawn given the same three angles