Solve the equation for x
dx + b = 9(x - d)

45% of the trees in a park are apple trees. there are 27 apple trees in the park. how many trees are in the park in all?

Set $r$ is a set of rectangles such that (1) only the grid points shown here are used as vertices, (2) all sides are vertical or horizontal and (3) no two rectangles in the set are congruent. if $r$ contains the maximum possible number of rectangles given these conditions, what fraction of the rectangles in set $r$ are squares? express your answer as a common fraction.

What is the point of inflection for the function f(x)=2cos4x on the interval (0, pi/2)