# Thread: Are we are allow to use L'Hospital's Rule for lim question

1. ## Are we are allow to use L'Hospital's Rule for lim question

So basically can we use L'Hospital's Rule for lim question (lim x>0 sinx/x) etc

2. No… that's just 1. You should just know that.

3. Also, L'hopital's rule isn't in the syllabus so maybe avoid it.

lol

5. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Originally Posted by TheBloop
Also, L'hopital's rule isn't in the syllabus so maybe avoid it.
+1.

6. ## Re: Are we are allow to use L'Hospital's Rule for lim question

I'd avoid it. I use it in school exams but for externals I'm not going to.

7. Originally Posted by RealiseNothing
I'd avoid it. I use it in school exams but for externals I'm not going to.
if you can't use an alternate method in school what makes you think you'll be able to for HSC?

8. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Originally Posted by Saturn WY15
So basically can we use L'Hospital's Rule for lim question (lim x>0 sinx/x) etc
Probably not. Otherwise it'd be in the syllabus.

9. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Originally Posted by Riproot
if you can't use an alternate method in school what makes you think you'll be able to for HSC?
I am allowed to use alternate methods in school.

10. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Originally Posted by Saturn WY15
So basically can we use L'Hospital's Rule for lim question (lim x>0 sinx/x) etc
In high school, it's sort of an identity that the limit of sin(x)/x = 1, and they say that due to small angle approximations. Since they treat it as an identity, you just need to manipulate similar questions to get it into the form of sin(x)/x or likewise.

But you can't use L'H for the HSC.

11. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Originally Posted by Riproot
if you can't use an alternate method in school what makes you think you'll be able to for HSC?
I think you are technically allowed to but its stupid because

a) As you and D94 have pointed out, there is an easy method in the syllabus which can be used
b) If your marker does not have the mathematical knowledge to understand the alternate methods you use, its likely they will assume you're talking out of your ass and give you minimal marks. In other words, its risky and tbh if you have the ability to comprehend and utilise out of syllabus mathematics, you should have the ability to solve those questions with the methods in the syllabus anyway

12. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Originally Posted by mirakon
I think you are technically allowed to but its stupid because

a) As you and D94 have pointed out, there is an easy method in the syllabus which can be used
b) If your marker does not have the mathematical knowledge to understand the alternate methods you use, its likely they will assume you're talking out of your ass and give you minimal marks. In other words, its risky and tbh if you have the ability to comprehend and utilise out of syllabus mathematics, you should have the ability to solve those questions with the methods in the syllabus anyway
Yeah i know but i just find it so much easier using the L'Hospital rule. Oh well ;0

13. ## Re: Are we are allow to use L'Hospital's Rule for lim question

In first year maths at sydney uni our lecturer told us not to use L'Hopital's rule for sinx/x because it is used when establishing d/dx(sinx), creating a circular argument. There is another way to set up the trig functions that bypasses the issue but it just might be worth noting.

14. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Another way is the taylor series expansion.

ie. sin x / x = (x-x^3/3!+x^5/5!- ....) / x = 1 - (x^2/3!-x^4/5!+...)

It is clear that if you set x->0 then sin x / x -> 1.

15. ## Re: Are we are allow to use L'Hospital's Rule for lim question

You shouldn't see any questions that require L'hospital's rule in 3U. Though Saturn's tutor said it can be used in 4U.

16. ## Re: Are we are allow to use L'Hospital's Rule for lim question

No, don't use it.

The HSC is there to test HOW WELL you can apply what you're meant to know, not just how much you know out-of-syllabus things (though of course that helps sometimes).

17. ## Re: Are we are allow to use L'Hospital's Rule for lim question

No - the questions involving these are easy enough!!

18. ## Re: Are we are allow to use L'Hospital's Rule for lim question

(is that a challenge?)

19. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Originally Posted by cutemouse
Another way is the taylor series expansion.

ie. sin x / x = (x-x^3/3!+x^5/5!- ....) / x = 1 - (x^2/3!-x^4/5!+...)

It is clear that if you set x->0 then sin x / x -> 1.
Yes because Taylor Series is part of the HSC...

20. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Haha carrot, no it is not, but by the trend of the questions that related to these identities (sinx/x, x/sinx, tanx/x, x/tanx) are quite straightforward.

21. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Originally Posted by Carrotsticks
(is that a challenge?)

22. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Haha trust nooblet94

23. ## Re: Are we are allow to use L'Hospital's Rule for lim question

I thought any theorem/technique should be allowed as long as you say what it is and use it correctly.

24. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Originally Posted by FBJ
I thought any theorem/technique should be allowed as long as you say what it is and use it correctly.
Pretty sure that if you want to use something outside the syllabus you have to derive it.

25. ## Re: Are we are allow to use L'Hospital's Rule for lim question

Yes because Taylor Series is part of the HSC...
umm its actually been in a few question 8's in 4unit over the years...
me and my cousin were doing this yesterday lol:P

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