California’s proposed new math curriculum defies logic

> Today, getting the correct answer to a math problem is still important. But teachers are also showing students that being “good at math” means applying that correct answer in service of a larger goal ...

> In math classes, an answer becomes a launching point for conversations about how and why people solved the problem in different ways. As we expand our understandings, teachers often welcome the problem-solving strategies that students bring from their homes and cultures in addition to familiar formal U.S. strategies. This means teachers must make space for students to explore why solutions work.

> To accomplish this, we encourage teachers to focus on math problems that invite curiosity and discussion ...

Here's the full vignette:

Vignette–36 Fences

Lori, a high school geometry teacher, introduces a problem to students. Lori explains that a farmer has 36 individual fences, each measuring one meter in length, and that the farmer wants to put them together to make the biggest possible area. Lori takes time to ask her students about their knowledge of farming, making reference to California’s role in the production of fruit, vegetables, and livestock. The students engage in an animated discussion about farms and the reasons a farmer may want a fenced area. While some of Lori’s long-term English learners show fluency with social/conversational English, she knows some will be challenged by forthcoming disciplinary literacy tasks. To support meaningful engagement in increasingly rigorous course work, she ensures images of all regular and irregular shapes are posted and labeled on the board, along with an optional sentence frame, “The fence should be arranged in a [blank] shape because [blank].” These support instruction when Lori asks students what shapes they think the fences could be arranged to form. Students suggest a rectangle, triangle, or square. With each response, Lori reinforces the word with the shape by pointing at the image of the shapes. When she asks, “How about a pentagon?” she reminds students of the optional sentence frame as they craft their response. Lori asks, the students think about this and talk about it as mathematicians. Lori asked them whether they want to make irregular shapes allowable or not.

After some discussion, Lori asks the students to think about the biggest possible area that the fences can make. Some students begin by investigating different sizes of rectangles and squares, some plot graphs to investigate how areas change with different side lengths.

Susan works alone, investigating hexagons––she works out the area of a regular hexagon by dividing it into six triangles and she has drawn one of the triangles separately. She tells Lori that she knew that the angle at the top of each triangle must be 60 degrees, so she could draw the triangles exactly to scale using compasses and find the area by measuring the height.

Niko has found that the biggest area for a rectangle with perimeter 36 is a 9 x 9 square—which gave him the idea that shapes with equal sides may give bigger areas and he started to think about equilateral triangles. Niko was about to draw an equilateral triangle when he was distracted by Jaden who told him to forget triangles, he had found that the shape with the largest area made of 36 fences was a 36-sided shape. Jaden suggested to Niko that he find the area of a 36-sided shape too and he leant across the table excitedly, explaining how to do this. He explained that you divide the 36-sided shape into triangles and all of the triangles must have a one-meter base, Niko joined in saying, “Yes, and their angles must be 10 degrees!” Jaden said, “Yes, and to work it out we need tangent ratios which Lori has just explained to me.” Jaden and Niko move closer together, incorporating ideas from trigonometry, to calculate the area.

As the class progressed many students started using trigonometry, some students were shown the ideas by Lori, some by other students. The students were excited to learn about trig ratios as they enabled them to go further in their investigations, they made sense to them in the context of a real problem, and the methods were useful to them. In later activities the students revisited their knowledge of trigonometry and used them to solve other problems.

To truly understand it, students have to go beyond and those concepts aren't available for them yet.

Step 1 would be to demonstrate that the shape with max area has to be convex. This is pretty straightforward, and a pretty decent example for advanced middle school kids.

Once you've got convexity proven, then the total area covered becomes the sum of triangular wedges. There are a variety of ways here to arrive at the maximum, either by a sequence of replacements (probably most appropriate for advanced middle schoolers) or a formulation in terms of Langrange multipliers (for advanced high school students).

I think a lot of students would find the proof pretty cool.

But it's obvious, and only obvious. I have no clue how to prove it. I could prove that if you're close to a circular shape, that shifting to more circular will increase the area. But that's not a complete proof.

Still, I have to say that this seems obvious enough to me that the difficulty in proving it doesn't really bother me too much. I would have to read the actual proposed standards to know whether these complaints are cherry-picking.

There's long been a battle between conceptual understanding and rote learning, and I lean towards conceptual understand and yet don't want it to go too far -- you need practice in order to cement things and to provide an opportunity to cover all the little subtle gaps that you won't fill in with a top-down approach. A student has to work actual concrete problems no matter how well they grasp the general concepts.