SG doesn't only stand for Singapore...

Algebra Level 4

Compute ( 1 0 4 + 324 ) ( 2 2 4 + 324 ) ( 3 4 4 + 324 ) ( 4 6 4 + 324 ) ( 5 8 4 + 324 ) ( 4 4 + 324 ) ( 1 6 4 + 324 ) ( 2 8 4 + 324 ) ( 4 0 4 + 324 ) ( 5 2 4 + 324 ) \frac{(10^4+324)(22^4+324)(34^4+324)(46^4+324)(58^4+324)}{(4^4+324)(16^4+324)(28^4+324)(40^4+324)(52^4+324)}

Please don't use a calculator. (AIME question)


The answer is 373.

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2 solutions

Karthik Kannan
Jul 6, 2014

The given product can be written as:

r = 0 4 ( 10 + 12 r ) 4 + 4 ( 3 ) 4 ( 4 + 12 r ) 4 + 4 ( 3 ) 4 \displaystyle\prod_{r=0}^{4}\frac{(10+12r)^{4}+4(3)^{4}}{(4+12r)^{4}+4(3)^{4}}

Now we shall use the Sophie Germain Identity

a 4 + 4 b 4 = ( a 2 + 2 b 2 + 2 a b ) ( a 2 + 2 b 2 2 a b ) a^{4}+4b^{4}=(a^{2}+2b^{2}+2ab)(a^{2}+2b^{2}-2ab)

to obtain:

r = 0 4 ( ( 10 + 12 r ) 2 + 2 ( 3 ) 2 + 2 × 3 × ( 10 + 2 r ) ) ( ( 10 + 12 r ) 2 + 2 ( 3 ) 2 2 × 3 × ( 10 + 2 r ) ) ( ( 4 + 12 r ) 2 + 2 ( 3 ) 2 + 2 × 3 × ( 4 + 2 r ) ) ( ( 4 + 12 r ) 2 + 2 ( 3 ) 2 2 × 3 × ( 4 + 2 r ) ) \displaystyle\prod_{r=0}^{4}\frac{\bigg( (10+12r)^{2}+2(3)^{2}+2\times 3\times (10+2r)\bigg)\bigg( (10+12r)^{2}+2(3)^{2}-2\times 3\times (10+2r)\bigg)}{\bigg( (4+12r)^{2}+2(3)^{2}+2\times 3\times (4+2r)\bigg)\bigg( (4+12r)^{2}+2(3)^{2}-2\times 3\times (4+2r)\bigg)}

= r = 0 4 ( 144 r 2 + 312 r + 178 ) ( 144 r 2 + 168 r + 58 ) ( 144 r 2 + 168 r + 58 ) ( 144 r 2 + 24 r + 10 ) =\displaystyle\prod_{r=0}^{4}\frac{(144r^{2}+312r+178)(144r^{2}+168r+58)}{(144r^{2}+168r+58)(144r^{2}+24r+10)}

= r = 0 4 ( 12 ( r + 1 ) + 1 ) 2 + 9 ( 12 r + 1 ) 2 + 9 =\displaystyle\prod_{r=0}^{4}\frac{(12(r+1)+1)^{2}+9} {(12r+1)^{2}+9}

= 6 1 2 + 9 1 2 + 9 =\dfrac{61^{2}+9}{1^{2}+9}

= 373 =\boxed{373}

How do you know that the equation can be written like that, from step one to step two

Trevor Arashiro - 6 years, 11 months ago

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A short derivation of the Sophie-Germain Identity is:

a 4 + 4 b 4 = a 4 + 4 a 2 b 2 + 4 b 4 4 a 2 b 2 = ( a 2 + 2 b 2 ) 2 ( 2 a b ) 2 = ( a 2 + 2 a b + 2 b 2 ) ( a 2 2 a b + 2 b 2 ) a^4+4b^4=a^4+4a^2b^2+4b^4-4a^2b^2=(a^2+2b^2)^2-(2ab)^2=(a^2+2ab+2b^2)(a^2-2ab+2b^2)

Cody Johnson - 6 years, 11 months ago

The Sophie-Germain Identity.

Joshua Ong - 6 years, 11 months ago

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Like how do we know that the given product can be written as that

Trevor Arashiro - 6 years, 11 months ago

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@Trevor Arashiro how u did the first step..plz explain it

praveen kumar - 6 years, 11 months ago

I meant the step before that

Trevor Arashiro - 6 years, 11 months ago

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@Trevor Arashiro Experience leads to good observations.

Joshua Ong - 6 years, 11 months ago

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@Joshua Ong Come to realize it, your right, I understand it now. That's a lot of math: pure observation.

Trevor Arashiro - 6 years, 11 months ago

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@Trevor Arashiro what do u think the minimum standard and age required to do this?

Suneel Kumar - 5 years, 7 months ago

You use the Sophie Germain Identity to convert the first step to the second step. (By substituting 10 + 12 r = a 10+12r = a and 3 = b 3 = b for the numerator and 4 + 12 r = a 4+12r = a and 3 = b 3 = b for the denominator)

Happy Melodies - 6 years, 11 months ago

how did you do the first step???????????????????

Sarfraz Ahmed Razi - 6 years, 11 months ago

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Realise that 10 = 10 + 12 ( 0 ) , 22 = 10 + 12 ( 1 ) , 34 = 10 + 12 ( 2 ) , 10 = 10+12(0), 22 = 10 + 12(1), 34 = 10 + 12(2), … that's how you get the 10 + 12 r 10 + 12r and r = 0 , 1 , 2 , 3 , 4 r = 0,1,2,3,4 in the pi product sign

Happy Melodies - 6 years, 11 months ago

Or rather step 0

Trevor Arashiro - 6 years, 11 months ago

awesome .!

Akshit Vashishth - 6 years, 11 months ago

What if I never knew sophai German equation

Can u solve in a difrent way or ways?

Sanoop Allaikal - 6 years, 10 months ago

I didn't know SG stands for SinGapore!! It stands for @Satvik Golechha and yes Sophie-Germain!!

Well, we can also do it like the way that this series telescope and so, it will be (3730/10) = 373

Kartik Sharma - 6 years, 10 months ago

how did you get 61^2 in the last step

devesh golwalkar - 6 years, 9 months ago

excellent solution

Viswakanth Kandala - 6 years, 8 months ago

A bit confusing considering that i haven't learned this yet. But are you sure you did not skip anything?

Leo Qiu - 6 years, 11 months ago

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yup... which part are u confused with? :)

Happy Melodies - 6 years, 11 months ago

good work battacharya

Narendar Bandaru - 5 years, 9 months ago

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