20050821, 05:22  #1 
Aug 2005
7 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/(31)]*Summation[i*3^3 * (31)]. thanks for your looking!!! 
20050821, 07:11  #2 
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. 
20050821, 10:43  #3 
Aug 2002
Termonfeckin, IE
2510_{10} 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. 
20050821, 11:15  #4 
"Nancy"
Aug 2002
Alexandria
2^{5}·7·11 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 
20050822, 02:24  #5 
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 :(

20050822, 03:50  #6  
Dec 2003
Hopefully Near M48
2·3·293 Posts 
Quote:
Admittedly, garo's and akruppa's solutions are better, since they don't involve this kind of "cheating". Last fiddled with by jinydu on 20050822 at 03:51 

20050830, 02:40  #7 
Aug 2005
7 Posts 
I don’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?

20050830, 03:14  #8  
Dec 2003
Hopefully Near M48
2·3·293 Posts 
Quote:
Last fiddled with by jinydu on 20050830 at 03:14 

20050830, 17:55  #9 
∂^{2}ω=0
Sep 2002
República de California
9837_{10} 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 stillsimmering scientific controversy. (Hey, if even one person in the whole world doesn't believe it, it is by definition controversial.)

20050830, 17:58  #10  
Banned
"Luigi"
Aug 2002
Team Italia
2^{2}·1,193 Posts 
Quote:
Why mirrors swap left and right, but don't swap up with down? Luigi 

20050830, 21:10  #11  
Bamboozled!
"𒉺𒌌𒇷𒆷𒀭"
May 2003
Down not across
3^{2}×11×103 Posts 
Quote:
Paul 

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