I know that a diagonal is a line segment that connects two non-adjacent vertices of a polygon. I also know that the number of diagonals of a convex n-gon is given by the formula n(n-3)/2, where n is the number of sides. So, to find the number of diagonals of a convex decagon, I just need to plug in n = 10 into the formula. I get 10(10-3)/2 = 10(7)/2 = 35. 
A square has two diagonals, and a convex pentagon has five diagonals.  How many diagonals does a convex decagon have?