MedVision ad

2013 CSSA Mathematics Thoughts? (1 Viewer)

coyney

Member
Joined
Mar 11, 2012
Messages
116
Gender
Male
HSC
2013
Hate to ruin people's days further, but Wagig pointed it out to me that the second 12hrs would be the next day... and so the sum would be 2x(that thing) - 12.

Made the same mistake :L
I'm pretty sure you don't take away 12, because you don't count the 12 for 12 am as the first ring... 1am + 2am + 3am + 4 + 5 +6 +7 +8 +9 +10 +11 +12pm + 1pm + ....... + 12 am
So i'm sure its 156
 

bedpotato

Member
Joined
Jul 10, 2013
Messages
337
Gender
Undisclosed
HSC
2013
Or equal to. Lol why would it be strictly greater than? What part of the solution excluded it equaling 1/2?
| r | > 1.
That's what I used to solve for y.
What did you do?
 
Last edited:

Triage

Member
Joined
May 30, 2012
Messages
245
Gender
Undisclosed
HSC
N/A
| r | > 1. Sorry, not < .
That's what I used to solve for y, where r = y^2/(y-1)^2.
What did you do?
I solved for infinite sum and did the opposite in the end
So -1<y^2/(y-1)^2 < 1

The -1 got eliminated because it has no real solutions.
 

dunjaaa

Active Member
Joined
Oct 10, 2012
Messages
473
Gender
Male
HSC
2014
The question was: for what values of y does this series does not have an infinite sum, I solved for r>1 and r<-1. r<-1 gave no solution and r>1 gave a solution of y>1/2. I think I got this question wrong since my friend pointed out to me that since the question implies that for this series to not have a infinite sum means that it has a limiting sum which meant you had to solve for r<1 and r>-1. Idk if thats correct or not but it sounds right. Lost 1 mark if he is right.
 

bedpotato

Member
Joined
Jul 10, 2013
Messages
337
Gender
Undisclosed
HSC
2013
The question was: for what values of y does this series does not have an infinite sum, I solved for r>1 and r<-1. r<-1 gave no solution and r>1 gave a solution of y>1/2. I think I got this question wrong since my friend pointed out to me that since the question implies that for this series to not have a infinite sum means that it has a limiting sum which meant you had to solve for r<1 and r>-1. Idk if thats correct or not but it sounds right. Lost 1 mark if he is right.
Yeah, I understand where you're coming from. But a limiting sum IS an infinite sum, it just has a limit. Lol, this is confusing me.
 

RealiseNothing

what is that?It is Cowpea
Joined
Jul 10, 2011
Messages
4,591
Location
Sydney
Gender
Male
HSC
2013
The question was: for what values of y does this series does not have an infinite sum, I solved for r>1 and r<-1. r<-1 gave no solution and r>1 gave a solution of y>1/2. I think I got this question wrong since my friend pointed out to me that since the question implies that for this series to not have a infinite sum means that it has a limiting sum which meant you had to solve for r<1 and r>-1. Idk if thats correct or not but it sounds right. Lost 1 mark if he is right.
Your friend is wrong.
 

RealiseNothing

what is that?It is Cowpea
Joined
Jul 10, 2011
Messages
4,591
Location
Sydney
Gender
Male
HSC
2013
The question was: for what values of y does this series does not have an infinite sum, I solved for r>1 and r<-1. r<-1 gave no solution and r>1 gave a solution of y>1/2. I think I got this question wrong since my friend pointed out to me that since the question implies that for this series to not have a infinite sum means that it has a limiting sum which meant you had to solve for r<1 and r>-1. Idk if thats correct or not but it sounds right. Lost 1 mark if he is right.
Also you are meant to solve for
 

iBibah

Well-Known Member
Joined
Jun 13, 2012
Messages
1,374
Gender
Male
HSC
2013
Yeah, I understand where you're coming from. But a limiting sum IS an infinite sum, it just has a limit. Lol, this is confusing me.
A limiting sum isn't an infinite sum. A limiting sum is a sum with infinite terms, however the sum of those terms is not infinity.

An infinite sum means the sum of the terms is infinite, it doesn't imply it has infinite terms (it DOES have infinite terms, but the name 'infinite sum' isn't so for that reason.

So while both limiting and infinite sums have infinite terms, they dont both have a sum that is infinity.
 

wagig

Member
Joined
Jan 29, 2013
Messages
152
Gender
Male
HSC
2015
If an infinite sum exists, that means there is a finite sum as the number of terms approaches infinity right?
 

bedpotato

Member
Joined
Jul 10, 2013
Messages
337
Gender
Undisclosed
HSC
2013
Also you are meant to solve for
How come?

A limiting sum isn't an infinite sum. A limiting sum is a sum with infinite terms, however the sum of those terms is not infinity.

An infinite sum means the sum of the terms is infinite, it doesn't imply it has infinite terms (it DOES have infinite terms, but the name 'infinite sum' isn't so for that reason.

So while both limiting and infinite sums have infinite terms, they dont both have a sum that is infinity.
Oh okay. Thanks.
 

RealiseNothing

what is that?It is Cowpea
Joined
Jul 10, 2011
Messages
4,591
Location
Sydney
Gender
Male
HSC
2013
A limiting sum isn't an infinite sum. A limiting sum is a sum with infinite terms, however the sum of those terms is not infinity.

An infinite sum means the sum of the terms is infinite, it doesn't imply it has infinite terms (it DOES have infinite terms, but the name 'infinite sum' isn't so for that reason.

So while both limiting and infinite sums have infinite terms, they dont both have a sum that is infinity.
No an infinite sum is just a sum of infinite terms.
 
Last edited:

Triage

Member
Joined
May 30, 2012
Messages
245
Gender
Undisclosed
HSC
N/A
Also you are meant to solve for
Oh yeah that's how I got it. Lol I was wondering how I got it then i realised, if a limiting sum is strictly less, then not a limiting sum must be equal to or greater than
 

Triage

Member
Joined
May 30, 2012
Messages
245
Gender
Undisclosed
HSC
N/A
When will you people realise. An infinite sum, is a sum (defined) as n goes to infinite. Lol how is that even a question. An infinite sum. Not an infinite infinite
 

wagig

Member
Joined
Jan 29, 2013
Messages
152
Gender
Male
HSC
2015
When will you people realise. An infinite sum, is a sum (defined) as n goes to infinite. Lol how is that even a question. An infinite sum. Not an infinite infinite
This^ (i hope haha)
 

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top