Is the sequence arithmetic? 15, 14.5, 14, 13.5, 13,… if so, give the common difference.

Koda is making birdhouses for a park. the instructions tell him how many of each wooden shape is needed for one birdhouse. complete each statement to adjust the number of shapes for each new situation if he follows the given instructions. to make 8 birdhouses, koda will trapezoids.  for koda to make birdhouses, he will need 12 rectangles. if koda uses 4 squares to make birdhouses, he will use triangles.