Inequality proofs in exams (1 Viewer)

[leehuan]

Since logic is obviously not taught, I was wondering if in the exam...

Would working backwards with (implied by) cost marks?

 

[InteGrand]

Since logic is obviously not taught, I was wondering if in the exam...

Would working backwards with cost marks?

If your steps in the proof are all reversible (so "<=>" type steps), then to prove an inequality, it is logically valid to start with the given to-prove inequality and simplify it down using such reversible steps to something that you know is clearly true (e.g. (x-y)^2 >= 0), and then immediately claim that the desired inequality is thus true.

However, unfortunately the HSC can be dodgy with these things, so it's probably safer for HSC purposes to actually not start at the desired inequality.

Do you have an example of some type of working you're referring to? If you meant something like:

x^2 > x

<== x > 1

(To show x^2 > x if x > 1 (obviously simplistic example for illustration purposes.))

then you'd probably be safer in the HSC to instead write these in the reverse order.

Usually the main motivation to actually start off with the given inequality is to use it to use ==> steps from it and try to reduce to something we know is true, and then hopefully be able to reverse the steps (in other words the steps are <=>). So usually when we do working for these sorts of HSC Q's where we started with the thing to be proved, we'll end up hoping to in fact have <=> steps. If the steps are indeed "<=>", then as said before it is logically valid, but HSC markers may penalise you. (Make sure you write the "<=>" between the steps if you use that method to make it explicit, but I still wouldn't use it in the HSC if possible, because the markers seem not to like it.)

 

[leehuan]

I full on worked backwards for this one



Because I got it down to

 

[InteGrand]

I full on worked backwards for this one



Because I got it down to

If your steps are reversible, then it is definitely logically fine.

 

[Paradoxica]

I full on worked backwards for this one



Because I got it down to

True by the power means inequality.

 

[Shadowdude]

True by the power means inequality.

alright then m8


why don't you give that as a "proof" and see what happens

 

[Paradoxica]

alright then m8
why don't you give that as a "proof" and see what happens

I'm not in Uni yet, and until that is the case, I can do what I want to for the time being.