help with math

DSC · 2005-08-21, 05:22

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!!!

jinydu · 2005-08-21, 07:11

Mathematica gives the answer:

3/4 * (1 - 3^n + 2*3^n)

You should be able to verify that using induction.

garo · 2005-08-21, 10:43

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.

akruppa · 2005-08-21, 11:15

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

DSC · 2005-08-22, 02:24

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 :(

jinydu · 2005-08-22, 03:50

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".

DSC · 2005-08-30, 02:40

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?

jinydu · 2005-08-30, 03:14

Quote:

Originally Posted by DSC

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?

The Law of Reflection...

ewmayer · 2005-08-30, 17:55

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.)

ET_ · 2005-08-30, 17:58

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

xilman · 2005-08-30, 21:10

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

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 (i3^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[i3^3 (3-1)]. thanks for your looking!!!

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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 :(

2005-08-30, 02:40	#7
DSC 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?

2005-08-30, 17:55	#9
ewmayer ∂²ω=0 Sep 2002 República 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.)