Building and critiquing a mathematical argument
Construct and present logical mathematical arguments involving multiple steps; critique others' reasoning about fractions, angles, or calculations and clearly explain errors or alternative methods
What a learner can do afterwards
- Prove that 3/4 > 2/3 using a common denominator argument and a visual model
- Find and explain an error in a long multiplication where a partial product was misaligned
- Present a chain of reasoning to show that angles in a triangle sum to 180° by tearing and arranging
1 · Read
When you disagree about who got more cake, show each step. A math argument is a chain of steps that leads to a true answer. Each step must follow from the step before it. You showed why 1/2 is bigger than 1/4 because fewer pieces means bigger pieces. If you can walk a friend through every step, you can find the step where their chain breaks.
Which is bigger, 5/8 or 3/4? Step 1: use 8 for both. Step 2: 5/8 uses 8, and 3/4 becomes 6/8. Step 3: compare 6 and 5. Step 4: the picture shows the 6/8 share the numbers describe, so both match. Step 5: 3/4 is greater than 5/8.
Here is that argument as a chain. Step 1: tear the three corners off any triangle. Step 2: line them up side by side along a straight edge. Step 3: they fit exactly, with no gap and no overlap. Step 4: a straight line measures 180 degrees. Step 5: so the three angles of any triangle add up to 180 degrees.
Ben multiplied 23 by 14. He wrote 23 x 4 = 92 first. For the tens digit, he wrote 23 x 1 = 23, then added 92 + 23 to get 115. But the 1 in 14 stands for 10, not 1, so that line should be 23 x 10 = 230. The correct total is 92 + 230 = 322. Ben's mistake was forgetting to shift the second line one place left.
When you check someone's work, don't just say it's wrong. Point to the exact step where the argument breaks, and explain what should happen instead. That helps far more than just saying 'wrong.'
A good argument moves step by step, and a good check finds the exact step that breaks.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
18 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.