What's Happening?

Shapes are added to other shapes. Can you see what is happening? What is the rule?

Problem

What's Happening? printable sheet

Image
A pentagon.

We started with this pentagon.

Added a triangle:

Image
The pentagon from the previous picture, with an equilateral triangle along the bottom right side.

Then another one:

Image
The previous image with an added triangle along the top side.

What would come next, and why?

Can you invent something like these for your friends to solve?