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 2005-08-21, 05:22 #1 DSC   Aug 2005 716 Posts help with math I have this problem but can't solve it! Can some one help?? Derive this expression interm of n "Summation (i*3^i); this summation is from i =1 to n " my prof. give a hint that multiply out side and inside the summation [1/(3-1)]*Summation[i*3^3 * (3-1)]. thanks for your looking!!!
 2005-08-21, 07:11 #2 jinydu     Dec 2003 Hopefully Near M48 2·3·293 Posts Mathematica gives the answer: 3/4 * (1 - 3^n + 2*3^n) You should be able to verify that using induction.
 2005-08-21, 10:43 #3 garo     Aug 2002 Termonfeckin, IE 53178 Posts After your prof's hint, try to see if it looks like a derivative. It's a homework problem so would appreciate it if folks limited it to hints and did not give away answers.
 2005-08-21, 11:15 #4 akruppa     "Nancy" Aug 2002 Alexandria 2,467 Posts You can do it without derivatives. Use your Prof's hint, split the sum into two sums, substitute the variable of summation in one sum suitably so both sums contain like powers of 3, join the two sums again and simplify. Alex
 2005-08-22, 02:24 #5 DSC   Aug 2005 7 Posts thanks alotz.....but jinydu, would you tell me how did you get 3/4 * (1 - 3^n + 2*3^n)......i've been trying to figure it out, but i couldn't find the answer anywayz :(
2005-08-22, 03:50   #6
jinydu

Dec 2003
Hopefully Near M48

175810 Posts

Quote:
 Originally Posted by DSC thanks alotz.....but jinydu, would you tell me how did you get 3/4 * (1 - 3^n + 2*3^n)......i've been trying to figure it out, but i couldn't find the answer anywayz :(
I used a computer program...

Admittedly, garo's and akruppa's solutions are better, since they don't involve this kind of "cheating".

Last fiddled with by jinydu on 2005-08-22 at 03:51

 2005-08-30, 02:40 #7 DSC   Aug 2005 7 Posts I donโt understand why I look into a mirror, and then a dimple in the right side of my face turns to opposite side. Could you explain why it is?
2005-08-30, 03:14   #8
jinydu

Dec 2003
Hopefully Near M48

2×3×293 Posts

Quote:
 Originally Posted by DSC I donโt understand why I look into a mirror, and then a dimple in the right side of my face turns to opposite side. Could you explain why it is?
The Law of Reflection...

Last fiddled with by jinydu on 2005-08-30 at 03:14

 2005-08-30, 17:55 #9 ewmayer ∂2ω=0     Sep 2002 Repรบblica de California 2DCF16 Posts Jinydu, you probably just gravely offended the adherents of some major religion which doesn't believe in the law of reflection. I think we need to overhaul science curricula worldwide to expose students to this still-simmering scientific controversy. (Hey, if even one person in the whole world doesn't believe it, it is by definition controversial.)
2005-08-30, 17:58   #10
ET_
Banned

"Luigi"
Aug 2002
Team Italia

22×7×173 Posts

Quote:
 Originally Posted by ewmayer Jinydu, you probably just gravely offended the adherents of some major religion which doesn't believe in the law of reflection. I think we need to overhaul science curricula worldwide to expose students to this still-simmering scientific controversy. (Hey, if even one person in the whole world doesn't believe it, it is by definition controversial.)
Yeah!

Why mirrors swap left and right, but don't swap up with down?

Luigi

2005-08-30, 21:10   #11
xilman
Bamboozled!

"๐บ๐๐ท๐ท๐ญ"
May 2003
Down not across

23·32·157 Posts

Quote:
 Originally Posted by ET_ Yeah! Why mirrors swap left and right, but don't swap up with down? Luigi
They swap neither. They swap front and back.

Paul

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