Given that a + b + c = 1 a 2 + b 2 + c 2 = 2 a 3 + b 3 + c 3 = 3 . If a 4 + b 4 + c 4 = a 2 a 1 where a 1 , a 2 are positive coprime integers, then find a 1 + a 2 .
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what about this solution:
consider a monic polynomial P ( x ) = x 3 + p x 2 + q x + r and let a , b , c , d be the roots of P ( x )
from the given, we have a b + b c + c a = − 2 1
By vietas relation, p = − 1 ; q = − 2 1
for a,b,c each, we have: z 3 − z 2 − 2 1 z + r = 0 where z = a , o r , b , o r , c
plugging a,b,c each and adding the three,and then putting values of given expressions we get: 3 − 2 − 2 1 = − 3 p
or p = − 6 1
so, Our polynomial P ( x ) = x 3 − x 2 − 2 1 a − 2 1
now, for z = a , o r , b , o r , c . . . . . . z n + 3 − z n + 2 − 2 1 z n + 1 − 2 1 z n = 0 ....... (multiplying the normal form by z n )
now, plug n = 1 and z = a , b , c resp. and add to get a 4 + b 4 + c 4 = 3 + 1 + 6 1
thus: giving: 6 2 5
answer: 3 1
believe me, even if this looks lengthy, it involves actually much, MUCH less calculation, the most of it is in your head basically :D
nice way out, did d the way.But I must admit that the solution was quite lengthy
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
Where we assume that a n is the leading coefficient of polynomial P ( x ) = a 3 x 3 + a 2 x 2 + a 1 x + a 0 .
After algebraic manipulations, we get that S ( 4 ) = 2 5 / 6 by a ( n − 4 ) = 0 since the polynomial is of third-degree and can hold only 4 coefficients.
As #Aditya Raut said, a^4+b^4+c^4 = 4 +1/6 = 25/6
thus,
a1 + a2 = 25 +6 = \boxed{31}
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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( 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 + b c + c a = 2 − 1
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 ) ..... i d e n t i t y
3 − 3 a b c = 2 + 2 1 ⟹ a b c = 6 1
( 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 )
4 1 = a 2 b 2 + b 2 c 2 + c 2 a 2 + 2 a b c ( a + b + c )
= a 2 b 2 + b 2 c 2 + c 2 a 2 + 3 1
Hence a 2 b 2 + b 2 c 2 + c 2 a 2 = 1 2 − 1
Now as we want 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 )
4 = a 4 + b 4 + c 4 − 6 1 ....as we obtained that value 1 2 − 1
Hence a 4 + b 4 + c 4 = 4 + 6 1 = 6 2 5
Hence answer is 3 1
I guess this solution gives the method to the whole series.... Try it out ! Good Luck