Limit problem (1 Viewer)

[KeypadSDM]

let's say I wanted to evaluate the limit:

lim[x-->0+] (-2x²Sec²[x])/(Tan[x])
And did this:

= lim[x-->0+] (-2Sec²[x]) * lim[x-->0+] (x²)/(Tan[x])

Is there anything wrong with that?

 

[redslert]

L'hopital's rule?

 

[KeypadSDM]

No, I'm asking if that step is legitamite. Obviously the limit is zero just by intuition (notice the coefficient of x/tan[x], which is 1, sec[x], which is 1, and a remaining x, which is zero)

 

[turtle_2468]

As long as the one you're taking out doesn't do dodgy stuff like go to 0 or infinity, I think you're fine. In this case, take out sec blah, which is a constant (ie 2)...

 

[KeypadSDM]

Yeah, and they marked me down in an assignment for it.

"Needs more explanation"

F*cking bastards, they've done this to me time and time again.

I DON'T WANT TO DUMB DOWN MY ANSWERS. THERE'S A REASON I GET THEM RIGHT.

 

[jm1234567890]

well, you don't really know if the thing your taking out is going to exist or not untill you have evaluated it.

So i guess you need to explain it.

 

[Affinity]

yeah some markers are a bit anal.

 

[turtle_2468]

But you need to EXPLAIN why it's a valid step. Without explanation many things could go down the drain... eg take a dodgy proof. The only way people could be convinced that your proof was right without you writing that bit down is by knowing that you NORMALLY would think of that step... which has a few pitfalls
1) You use up your goodwill quite quickly
2) If you normally wouldn't think of that step, they wouldn't give you the mark and would think you're fudging. Which means they're discriminating based on students and not assignments, which is dodgy...

I reckon that step was sufficiently non-obvious if part of an assignment to warrant at least a short explanation... doesn't matter whether you get it right, it's the proof that counts

 

[jm1234567890]

^^

do you still do maths in medicine?

 

[KeypadSDM]

It's so annoying though.

Plus they marked down in assignments because they forgot to ask a question.

MATH1902 - http://www.maths.usyd.edu.au:8000/u/UG/JM/MATH1902/r/ass204.pdf

Check out question 1 part ii). They asked for the values of alpha, so I gave it to them, and they gave me 1/2 because I didn't explain/justify.

Well thanks buddy, next time ASK FOR JUSTIFICATION.

 

[turtle_2468]

JM...:Yeah, 3 units... which isn't much, but it's better than nothing. Plus it keeps my brain ticking over...
well, keypad, 1/2 a mark... don't worry yourself overly about it. Non-standard solutions are always tricky to mark anyway... but it's a bit of an art to figure out where others might have trouble understanding your proof. GOod luck for your next assignment, I have cramming to do...

 

[KeypadSDM]

I've learnt that the best way to do everything is to treat the marker like a 2 year old. Spoon feed everything, copy proofs. Don't assume they know anything that can be assumed in the course. Then you CAN'T lose a mark.

 

[Estel]

Nice to know I won't feel any culture shock in university...

 

[KeypadSDM]

I was hoping for a tad more acceptance here. It sucks the balls. They're seriously putting me off maths.