Instructions: 1. Right-click on one of the planes, and while pressing down on your mouse (or trackpad), rotate the planes to see how the figure looks like from different angles by moving your mouse (or finger on your trackpad). 2. Let go of your cursor, and deselect the green & red planes by clicking on the corresponding circles in the left menu. Notice now that the two planes are hidden, and observe how the line of intersection between the green and red planes (the black line) intersects the blue plane.
In this case, how does the line of intersection between the green and red planes intersect the blue plane? Hence, what conclusion can you draw from how we can determine the number of common points of intersection between all three planes, by investigating how the line of intersection between any of the two planes intersect the last plane?