ISF Logo   IS Forum
Forum Index Register Members List Events Mark Forums Read Help

Go Back   International Skeptics Forum » General Topics » Science, Mathematics, Medicine, and Technology
 


Welcome to the International Skeptics Forum, where we discuss skepticism, critical thinking, the paranormal and science in a friendly but lively way. You are currently viewing the forum as a guest, which means you are missing out on discussing matters that are of interest to you. Please consider registering so you can gain full use of the forum features and interact with other Members. Registration is simple, fast and free! Click here to register today.
Reply
Old 1st December 2017, 11:47 PM   #1
jt512
Graduate Poster
 
Join Date: Sep 2011
Posts: 1,673
Math experts: What is the value of this expression?

This is from a mathematics qualification pre-test from an on-line physics course. I'm not sure I agree with the official answer. I'm curious what you all think?

What is the value of the following expression?

jt512 is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 1st December 2017, 11:56 PM   #2
WhatRoughBeast
Graduate Poster
 
Join Date: Apr 2011
Posts: 1,239
Originally Posted by jt512 View Post
This is from a mathematics qualification pre-test from an on-line physics course. I'm not sure I agree with the official answer. I'm curious what you all think?

What is the value of the following expression?

The first summation gives (j + k + 3) + (j + k + 4) = 2j + 2k +7

The second gives (2j + 4 + 7) + ( 2j + 6 + 7) = 4j + 24

and the last gives (4 + 24) + (8 + 24) = 60
WhatRoughBeast is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd December 2017, 12:01 AM   #3
Clive
Critical Thinker
 
Join Date: Dec 2008
Posts: 364
Python agrees:

Code:
>>> total = 0
>>> for j in (1,2):
...    for k in (2,3):
...       for l in (3,4):
...          total += j + k + l
... 
>>> total
60

Last edited by Clive; 2nd December 2017 at 12:03 AM. Reason: put code in code block!
Clive is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd December 2017, 12:07 AM   #4
John Jones
Penultimate Amazing
 
John Jones's Avatar
 
Join Date: Apr 2009
Location: Iowa USA
Posts: 11,858
So you're saying that Jabba is numerically immortal?
__________________
"I have no clue" - King of the Americas

Last edited by John Jones; 2nd December 2017 at 12:11 AM.
John Jones is online now   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd December 2017, 12:23 AM   #5
Brainster
Penultimate Amazing
 
Brainster's Avatar
 
Join Date: May 2006
Posts: 14,666
Originally Posted by jt512 View Post
This is from a mathematics qualification pre-test from an on-line physics course. I'm not sure I agree with the official answer. I'm curious what you all think?

What is the value of the following expression?

I'm going to say that the sum j+k+l for l=3 to 4 boils down to:

2j+2k+7

If we sum that for k=2 to 3 we get:

2j+4+7+2j+6+7

Or 4j+24

Now summing for j=1 to 2 we get:

28+32=60
__________________
My new blog: Recent Reads.
1960s Comic Book Nostalgia
Visit the Screw Loose Change blog.
Brainster is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd December 2017, 12:25 AM   #6
Clive
Critical Thinker
 
Join Date: Dec 2008
Posts: 364
Shortest answer?

Averages of each range (1-2, 2-3, 3-4) are 1.5, 2.5, 3.5. Sum of those is 7.5. Eight combinations summed and 8 x 7.5 = 60.
Clive is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd December 2017, 12:14 PM   #7
Beelzebuddy
Philosopher
 
Beelzebuddy's Avatar
 
Join Date: Jun 2010
Posts: 5,678
There's three sigma symbols in the equation, and twenty symbols total. 3*20 = 60.
Beelzebuddy is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd December 2017, 04:42 PM   #8
Clive
Critical Thinker
 
Join Date: Dec 2008
Posts: 364
Originally Posted by Beelzebuddy View Post
There's three sigma symbols in the equation, and twenty symbols total. 3*20 = 60.
Funny, but let's say we change the ranges for each sigma so that instead of 1-2, 2-3, and 3-4 we're summing over (say) 1-5, 3-7, and 5-9.

Averages for the three new ranges are 3, 5, 7 which sum to 15. And there are 5 x 5 x 5 = 125 combinations. 125 x 15 = 1875.

Using averages like this works because fundamentally everything we'e doing is addition and addition is also commutative, and finally the sigma operation is spanning a uniform range of values...

But there are still 20 symbols. Hmm.... waiting for your new "answer"! ;-)
Clive is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd December 2017, 04:50 PM   #9
Hans
Philosopher
 
Hans's Avatar
 
Join Date: May 2007
Posts: 6,708
Originally Posted by jt512 View Post
This is from a mathematics qualification pre-test from an on-line physics course. I'm not sure I agree with the official answer. I'm curious what you all think?

What is the value of the following expression?

May I ask a historical question?

The symbols in that mathematical equation when would they have come into existence? Or put another way how far back in time could you go and still have an educated man understand that equation?

I'm asking because I am thinking of stealing that for a book but it is set in 1839 and would that be possible to write at that time?

Thanks
Hans is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 2nd December 2017, 05:35 PM   #10
Clive
Critical Thinker
 
Join Date: Dec 2008
Posts: 364
According to wikipedia (https://en.wikipedia.org/wiki/Histor...tical_notation) Euler used capital sigma for summation, and Euler lived in the 18th century so it seems you would be in good company to be using that notation in 1839. Euler was born in 1707 and died in 1783.
Clive is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 3rd December 2017, 12:40 AM   #11
Hans
Philosopher
 
Hans's Avatar
 
Join Date: May 2007
Posts: 6,708
Originally Posted by Clive View Post
According to wikipedia (https://en.wikipedia.org/wiki/Histor...tical_notation) Euler used capital sigma for summation, and Euler lived in the 18th century so it seems you would be in good company to be using that notation in 1839. Euler was born in 1707 and died in 1783.
Thanks - math history is not one of the things I've concentrated on, I would not have known how to look that up!

Last edited by Hans; 3rd December 2017 at 12:46 AM.
Hans is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 3rd December 2017, 12:51 AM   #12
psionl0
Skeptical about skeptics
 
psionl0's Avatar
 
Join Date: Sep 2010
Location: 31°57'S 115°57'E
Posts: 11,780
Originally Posted by Clive View Post
Python agrees:

Code:
>>> total = 0
>>> for j in (1,2):
...    for k in (2,3):
...       for l in (3,4):
...          total += j + k + l
... 
>>> total
60

Pshaw! If it isn't written in GW-BASIC then it isn't true.


ETA If you want to do the sum by hand then the working out is
(1+2+3) + (1+2+4) + (1+3+3) + (1+3+4) + (2+2+3) + (2+2+4) + (2+3+3) + (2+3+4) = 60
__________________
"The process by which banks create money is so simple that the mind is repelled. Where something so important is involved, a deeper mystery seems only decent." - Galbraith, 1975

Last edited by psionl0; 3rd December 2017 at 01:03 AM.
psionl0 is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Old 3rd December 2017, 06:29 AM   #13
Myriad
Hyperthetical
 
Myriad's Avatar
 
Join Date: Nov 2006
Location: Pennsylvania
Posts: 13,168
Originally Posted by jt512 View Post
What is the value of the following expression?

If I really needed it for something, I suppose I'd pay a few bucks for it.
__________________
A zømbie once bit my sister...
Myriad is offline   Quote this post in a PM   Nominate this post for this month's language award Copy a direct link to this post Reply With Quote Back to Top
Reply

International Skeptics Forum » General Topics » Science, Mathematics, Medicine, and Technology

Bookmarks

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -7. The time now is 05:40 PM.
Powered by vBulletin. Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
© 2014, TribeTech AB. All Rights Reserved.
This forum began as part of the James Randi Education Foundation (JREF). However, the forum now exists as
an independent entity with no affiliation with or endorsement by the JREF, including the section in reference to "JREF" topics.

Disclaimer: Messages posted in the Forum are solely the opinion of their authors.