Thursday, January 14, 2010

Near Miss

Yesterday at the start of geometry class I had the homework answers projected from the document camera, and was silently reviewing in my mind what we would be doing that day while my students checked their answers. I was ready to perfunctorily ask if they had any questions before we moved on to "the real learning". Okay, I'm not that robotic, but on hindsight, that's what it feels like because I almost missed a great learning moment.

On one particular question a bunch of students raised their hands. The question was something to the effect of: In a pentagon, the angles are consecutive multiples of 4. Find the measure of each angle. Instead of simply putting up the answer, I put my work down also since I thought there would be several confused students that would want it broken down: 4x + 4(x+1) + 4(x+2) + 4(x+3)+ 4(x+4) = 540 and then went from there.

The students with their raised hands weren't confused, they were chomping at the bit to show how they did the problem. Oh my goodness were there some clever kiddies there. They love to come up to the overhead and present (note to self: stop being a projector hog), and one by one they showed us what they did.

1st student: well, I divided 540 by 5 and got 108. That's a multiple of 4. The angles couldn't all be 108, but I thought they'd hover around it, so 108 was an angle, then for 2 other angles, I added and subtracted 4 and had 3 angles: 104, 108, 112. Then I added 4 to 112 and subtracted 4 from 104 to get my angles: 100,104,108,112,116.

2nd student: well, I started with 100 degrees for each angle (a multiple of 4), and with 5 angles that adds up to 500, so I had 40 degrees left to play with. Then I tried to break down 40 into 5 consecutive multiples of 4. 4, 8, 12, 16, 20. I added these up, and they were more than 40, so I just used 4,8,12,16 and it worked.

3rd student: well, I divided 540 by 5 and got 108. I divided 108 by 4 and saw that 108=27 * 4. So I then I found the consecutive numbers around 27: 25,26,27,28,29, and those were the consecutive multiples of 4, so I multiplied by those and got the answer.

4th student: well, I knew they were consecutive multiples of 4, so I set up an equation:
x + (x+4) + (x+8) + (x+12) + (x+16) = 540 and solved that.

I was very impressed and humbly reminded that there are teaching moments quite available to me even when I'm not "teaching".


  1. Oh, how I miss teaching those kinds of students!

  2. I know just what you mean. The whole class dynamic changes with the change in clientele.

    Then I have my algebra 1 classes where too many kids are still struggling with 4 - (-7).

    Ms. Cookie

  3. This is one reason I almost never present homework problems anymore. I always put up five or six problems on the board and have students volunteer to do their work on the board. Sometimes I have them explain their steps, sometimes I talk because there's something in particular I want to point out. This way, the students that didn't have trouble with the problems have something to do while we review problems and the students who need more help can take the time. We cover more problems this way and I get the chance to walk around the room and talk individually with students. I love reviewing homework this way.

  4. Ms. Ashton,

    I love your idea. I keep stressing about the time issue, though. How long does this take you on average?


    Ms. Cookie