Tilting towards Nrichment

TILTED SQUARES from the inspirational UK site nrich.maths.org is a great example of an open-ended math problem that leads to some deep math thinking… even among students who don’t normally show much interest in their math lesson.

The task at first sight appears to be quite simple. A square drawn on square-dotted paper is tilted by raising the lower right-hand corner vertically by one dot, and a new square is then drawn on this tilted base. The question is, what is the area of the new square? And then, is there a pattern to the areas with continuing tilts, as the lower right-hand corner is raised by a dot at a time? The immediate response of almost all the students in my classes was, the first square and the second square were exactly the same. This, even from my most accomplished mathematical thinkers. The Nrich site has an interactive program, so the square can be repeatedly tilted and projected. Even on tilt three or four students were still insistent the areas remained the same. Only when the square had tilted to become what students call a diamond, was there questioning.

My two most inquisitive students held out until after they’d carefully drawn and cut out the first two squares and placed one on top of the other before they would countenance any change.

Problems from Nrich seem to self-differentiate: two students who rarely are able to engage in more traditional exercises, quickly constructed on dotted paper a pattern of 18 tilted squares. They were excited. They were animated. And they wanted to talk about, and show what they had done. Others insisted on constructing the pattern on the interactive whiteboard. They did this while some of my more “analytical” students struggled to visualize and draw the squares, unable to identify the corners of a newly-tilting square.

Some students explored finding the areas of the square by measuring, and others tried Pythagoras.

The first found the results frustrating as they estimated fractions of a millimeter change in length, squaring the answers and ending up with lots of decimals and no clear pattern. The Pythagoreans built up tables and areas and tried to spot a pattern in the growth. They quickly spotted a pattern which they could verbally explain and extend, but could not or would not commit their analysis to paper, other than to list the answers.

Students were encouraged to try with a differently-sized first square. With a smaller starting square, the growth in area is more quickly apparent. The Pythagoreans could verbally list the areas with ease. But they refused to countenance any search for a method that could be committed to paper.

The majority drew several squares, and tried to estimate the area by counting squares formed by the dots — or just counting dots as an estimate. They could not extend to the results of the Pythagoreans, but were much more open to watching some hints on the interactive whiteboard: Why not a square with a horizontal base around the outside of any tilted square?

The area of the outside square, subtract four triangles and you have the area of the tilted square. The visual approach appealed to our early pattern artists, and provided the clue for the majority who were searching for a calculation.

Quickly more areas were forthcoming, while the Pythagoreans reluctantly drew some titled squares with exterior square… only to dismiss the approach with open scorn.

What if there are 99-tilts? The Pythagoreans raced for the answer, but could not agree.

Meanwhile another hint: Build up a table breaking each area calculation down into all of its detailed parts, including some reference to the tilt number. Use different colors for the numbers from different parts of the calculation, red for the tilt number, green for the side length of the outside square and so on. Is there a pattern?

What if the tilt number is n, representing the nth tilt? Can n be identified in the pattern?

The early doubters confidently give the answer for the area of the 99th tilt. The Pythagoreans are still arguing about the mental math, though there are signs on paper of the formula that was, indeed, in their heads.

And the artists had now produced colored titled squares and were demanding their works were now put on display.

Now that’s enriched mathematics. And nothing feels so Good!

❏ For more discussion about using open-ended questions to promote mathematical thinking… plus thoughts on what exactly constitutes math thinking see About Nrich: research plus articles