- #1
cerealkiller
- 7
- 0
Hi guys,
I have to teach inequality proofs and am looking for an opinion on something.
Lets say I have to prove that a2+b2≥2ab. (a very simple example, but I just want to demonstrate the logic behind the proof that I am questioning)
Now the correct response would be to start with the inequality (a - b)2≥0, then progress to:
a2+b2-2ab≥0
∴ a2+b2≥2ab.
What many students do, as it is generally much easier, is to start with the required result and work backwards:
i.e.
a2+b2≥2ab
a2+b2-2ab≥0
∴ (a - b)2≥0, which is true, ∴ the initial result must also be true.
Can anyone provide an example where starting with the result and working toward a true statement will not work? My colleagues would generally discourage this approach but it would be much more convincing to students if I could show them a situation where it won't work.
Thanks for the help guys!
I have to teach inequality proofs and am looking for an opinion on something.
Lets say I have to prove that a2+b2≥2ab. (a very simple example, but I just want to demonstrate the logic behind the proof that I am questioning)
Now the correct response would be to start with the inequality (a - b)2≥0, then progress to:
a2+b2-2ab≥0
∴ a2+b2≥2ab.
What many students do, as it is generally much easier, is to start with the required result and work backwards:
i.e.
a2+b2≥2ab
a2+b2-2ab≥0
∴ (a - b)2≥0, which is true, ∴ the initial result must also be true.
Can anyone provide an example where starting with the result and working toward a true statement will not work? My colleagues would generally discourage this approach but it would be much more convincing to students if I could show them a situation where it won't work.
Thanks for the help guys!