Bashing unavailable - part 1

Algebra Level 4

Given that a + b + c = 1 a 2 + b 2 + c 2 = 2 a 3 + b 3 + c 3 = 3 . \color{#20A900}{a+b+c =1} \\ \color{#3D99F6}{a^2+b^2+c^2=2} \\ \color{#D61F06}{a^3+b^3+c^3=3}. If a 4 + b 4 + c 4 = a 1 a 2 a^4+b^4+c^4= \frac{a_1}{a_2} where a 1 a_1 , a 2 a_2 are positive coprime integers, then find a 1 + a 2 a_1+a_2 .


The answer is 31.

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4 solutions

Aditya Raut
Jun 7, 2014

( a + b + c ) 2 = 1 = a 2 + b 2 + c 2 + 2 ( a b + b c + a c ) = 2 + 2 ( a b + b c + c a ) (a+b+c)^2 = 1= a^2+b^2+c^2 +2(ab+bc+ac)=2 +2(ab+bc+ca)

a b + b c + c a = 1 2 ab+bc+ca = \frac{-1}{2}

a 3 + b 3 + c 3 3 a b c = ( a + b + c ) ( a 2 + b 2 + c 2 a b b c c a ) a^3+b^3+c^3-3abc = (a+b+c)(a^2+b^2+c^2-ab-bc-ca) ..... i d e n t i t y \color{#D61F06}{identity}

3 3 a b c = 2 + 1 2 a b c = 1 6 3-3abc= 2+\frac{1}{2} \implies abc = \frac{1}{6}

( a b + b c + c a ) 2 = a 2 b 2 + b 2 c 2 + c 2 a 2 + 2 ( a b 2 c + a b c 2 + a 2 b c ) (ab+bc+ca)^2= a^2b^2+b^2c^2+c^2a^2+2(ab^2c+abc^2+a^2bc)

1 4 = a 2 b 2 + b 2 c 2 + c 2 a 2 + 2 a b c ( a + b + c ) \frac{1}{4} = a^2b^2+b^2c^2+c^2a^2 +2abc(a+b+c)

= a 2 b 2 + b 2 c 2 + c 2 a 2 + 1 3 = a^2b^2+b^2c^2+c^2a^2 +\frac{1}{3}

Hence a 2 b 2 + b 2 c 2 + c 2 a 2 = 1 12 a^2b^2+b^2c^2+c^2a^2 = \frac{-1}{12}

Now as we want a 4 + b 4 + c 4 a^4+b^4+c^4 we'll square the second expression.

( a 2 + b 2 + c 2 ) 2 = a 4 + b 4 + c 4 + 2 ( a 2 b 2 + b 2 c 2 + c 2 a 2 ) (a^2+b^2+c^2)^2= a^4+b^4+c^4 +2(a^2b^2+b^2c^2+c^2a^2)

4 = a 4 + b 4 + c 4 1 6 4=a^4+b^4+c^4 -\frac{1}{6} ....as we obtained that value 1 12 \frac{-1}{12}

Hence a 4 + b 4 + c 4 = 4 + 1 6 = 25 6 a^4+b^4+c^4=4+ \frac{1}{6} = \frac{25}{6}

Hence answer is 31 \boxed{31}

I guess this solution gives the method to the whole series.... Try it out ! Good Luck

what about this solution:

consider a monic polynomial P ( x ) = x 3 + p x 2 + q x + r P(x)=x^{3}+px^{2}+qx+r and let a , b , c , d a,b,c,d be the roots of P ( x ) P(x)

from the given, we have a b + b c + c a = 1 2 ab+bc+ca=-\frac{1}{2}

By vietas relation, p = 1 ; q = 1 2 p=-1;q=-\frac{1}{2}

for a,b,c each, we have: z 3 z 2 1 2 z + r = 0 z^{3}-z^{2}-\frac{1}{2}z+r=0 where z = a , o r , b , o r , c z=a,or,b,or,c

plugging a,b,c each and adding the three,and then putting values of given expressions we get: 3 2 1 2 = 3 p 3-2-\frac{1}{2}=-3p

or p = 1 6 p=-\frac{1}{6}

so, Our polynomial P ( x ) = x 3 x 2 1 2 a 1 2 P(x)=x^{3}-x^{2}-\frac{1}{2}a-\frac{1}{2}

now, for z = a , o r , b , o r , c . . . . . . z n + 3 z n + 2 1 2 z n + 1 1 2 z n = 0 z=a,or,b,or,c...... z^{n+3}-z^{n+2}-\frac{1}{2}z^{n+1}-\frac{1}{2}z^{n}=0 ....... (multiplying the normal form by z n z^{n} )

now, plug n = 1 n=1 and z = a , b , c z=a,b,c resp. and add to get a 4 + b 4 + c 4 = 3 + 1 + 1 6 a^{4}+b^{4}+c^{4}=3+1+\frac{1}{6}

thus: giving: 25 6 \frac{25}{6}

answer: 31 \boxed{31}

believe me, even if this looks lengthy, it involves actually much, MUCH less calculation, the most of it is in your head basically :D

Aritra Jana - 6 years, 8 months ago

nice way out, did d the way.But I must admit that the solution was quite lengthy

Divyanshu Vadehra - 6 years, 9 months ago

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Enjoy the process , it won't be lengthy.

Raven Herd - 6 years, 4 months ago

Using Newton's Identity where:

a n S 1 + a n 1 = 0 a n S 2 + a n 1 S 1 + 2 a n 2 = 0 a n S 3 + a n 1 S 2 + a n 2 S 1 + 3 a n 3 = 0 a n S 4 + a n 1 S 3 + a n 2 S 2 + a n 3 S 1 + 4 a n 4 = 0 a_nS_1 + a_{n-1} = 0 \\ a_nS_2 + a_{n-1}S_1 + 2a_{n-2} = 0 \\ a_nS_3 + a_{n-1}S_2 + a_{n-2}S_1 + 3a_{n-3} = 0 \\ a_n S_4 + a_{n-1}S_3 + a_{n-2}S_2 + a_{n-3}S_1 + 4a_{n-4} = 0

Where we assume that a n a_n is the leading coefficient of polynomial P ( x ) = a 3 x 3 + a 2 x 2 + a 1 x + a 0 P(x)=a_3x^3 + a_2x^2 + a_1x + a_0 .

After algebraic manipulations, we get that S ( 4 ) = 25 / 6 S(4) = 25/6 by a ( n 4 ) = 0 a(n-4) = 0 since the polynomial is of third-degree and can hold only 4 coefficients.

Kevin Patel
Jul 22, 2014

As #Aditya Raut said, a^4+b^4+c^4 = 4 +1/6 = 25/6
thus,
a1 + a2 = 25 +6 = \boxed{31}

Mriganka Bhatta
Sep 7, 2014

a^4+b^4+c^4=(a^2+b^2+c^2)^2- 2(a^2b^2+b^2c^2+c^2a^2) =4-2(a^2b^2+b^2c^2+c^2a^2) so i am in need of the value 2(a^2b^2+b^2c^2+c^2a^2). using (a+b+c)^3=1, I get abc =1/6.

and using the formula x^2+y^2+z^2=(x+y+z)^2-2(xy+yz+zx)

i get 2(a^2b^2+b^2c^2+c^2a^2)= -1/6 and a^4+b^4+c^4=4+1/6=25/6....... and the sum is 31..... thanks

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