# Math Is Fun Forum

Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫  π  -¹ ² ³ °

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## #1 2006-12-13 21:48:50

RauLiTo
Member
From: Bahrain
Registered: 2006-01-11
Posts: 142

### inner product

hi guys ... how are you ?!

if you know that : || L || = 4 ... || 2 L + M || = 13 ... the angle between the two vectors is θ = 120 ... what is the inner product of these two vectors ?! it looks easy but i dont know why they told me that my answer is wrong which was - 10 or 42

ImPo\$\$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

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## #2 2006-12-14 04:09:09

luca-deltodesco
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Registered: 2006-05-05
Posts: 1,470

### Re: inner product

inner product = dot product i think in which case

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## #3 2006-12-14 04:44:54

RauLiTo
Member
From: Bahrain
Registered: 2006-01-11
Posts: 142

### Re: inner product

no man i meant with the two vectors L and M ... we have L but we have to get M first

ImPo\$\$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

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## #4 2006-12-14 04:52:31

luca-deltodesco
Member
Registered: 2006-05-05
Posts: 1,470

### Re: inner product

oooh, you mean the product of L and M, not L and 2L+M

well then from that same equation i had

Last edited by luca-deltodesco (2006-12-14 04:54:28)

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## #5 2006-12-14 05:11:56

RauLiTo
Member
From: Bahrain
Registered: 2006-01-11
Posts: 142

### Re: inner product

theta is the angle between L and M so we cant use it to get the first equation you said

ImPo\$\$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

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## #6 2006-12-14 07:08:23

luca-deltodesco
Member
Registered: 2006-05-05
Posts: 1,470

### Re: inner product

ah... lol

Last edited by luca-deltodesco (2006-12-14 07:09:03)

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## #7 2006-12-14 16:32:40

RauLiTo
Member
From: Bahrain
Registered: 2006-01-11
Posts: 142

### Re: inner product

guys ?!

ImPo\$\$!BLe = NoTH!nG
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## #8 2006-12-14 17:32:35

Stanley_Marsh
Member
Registered: 2006-12-13
Posts: 345

### Re: inner product

I think you can draw a graph about it , 2L+M connect the two point of a parallelogram consists by M and 2L , knowing the length of 2L , and 2L+M ,and the angle , use geometry to solve it

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## #9 2006-12-15 06:50:40

RauLiTo
Member
From: Bahrain
Registered: 2006-01-11
Posts: 142

### Re: inner product

the final answer is - 30 ... anybody ?

ImPo\$\$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

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## #10 2006-12-15 07:19:24

luca-deltodesco
Member
Registered: 2006-05-05
Posts: 1,470

### Re: inner product

ok so we have

//+ is taken over -, since - would give a negative length to M, which is not allowed

Last edited by luca-deltodesco (2006-12-15 20:05:45)

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## #11 2006-12-15 20:03:13

RauLiTo
Member
From: Bahrain
Registered: 2006-01-11
Posts: 142

### Re: inner product

thaaaaanks btw its not 106 its 105
thans again

ImPo\$\$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

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## #12 2006-12-15 20:04:51

luca-deltodesco
Member
Registered: 2006-05-05
Posts: 1,470

### Re: inner product

RauLiTo wrote:

thaaaaanks btw its not 106 its 105
thans again

i corrected in my working out

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