Squares, Then Cubes, Now Squared Squares

Algebra Level 3

If a + b + c = 6 a+b+c=6 , a 2 + b 2 + c 2 = 14 a^2 +b^2+c^2=14 and a 3 + b 3 + c 3 = 36 a^3+b^3+c^3=36 , find the value of

a 4 + b 4 + c 4 . a^4+b^4+c^4.

99 96 98 100 97

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5 solutions

Eli Ross Staff
Feb 11, 2016

(This is not a recommended approach to the problem...)

"Hmm...I wonder if they are integers..."

"Well, what about 1, 2, and 3?"

"Oh, hmm, those work. Cool."

1 4 + 2 4 + 3 4 = 1 + 16 + 81 = 98. 1^4 +2^4 + 3^4 = 1 + 16 + 81 = 98.

Who would have thought?!!

Achal Jain - 5 years, 4 months ago

I solved it the same way!!

Ashish Menon - 5 years, 3 months ago
Mateus Gomes
Feb 7, 2016

a 2 + b 2 + c 2 = ( a + b + c ) 2 2 ( a b + a c + b c ) = ( 6 ) 2 2 ( a b + a c + b c ) = 14 ( a b + a c + b c ) = 11 a^2 +b^2+c^2=(a+b+c)^2-2(ab+ac+bc)=(6)^2-2(ab+ac+bc)=14\rightarrow\color{#20A900}{\boxed{(ab+ac+bc)=11}} a 3 + b 3 + c 3 = ( a + b + c ) 3 3 [ ( a + b + c ) ( a b + a c + b c ) a b c ] = ( 6 ) 3 3 [ ( 6 ) ( 11 ) a b c ] = 36 ( a b c ) = 6 a^3+b^3+c^3=(a+b+c)^3-3[(a+b+c)(ab+ac+bc)-abc]=(6)^3-3[(6)(11)-abc]=36\rightarrow\color{#69047E}{\boxed{(abc)=6}} a 4 + b 4 + c 4 = ( a + b + c ) 4 12 ( a b c ) ( a + b + c ) 4 [ a 3 ( c + b ) + b 3 ( c + a ) + c 3 ( a + b ) ] 6 [ ( a b + a c + b c ) 2 2 a b c ( a + b + c ) ] a^4+b^4+c^4=(a+b+c)^4-12(abc)(a+b+c)-4[a^3(c+b)+b^3(c+a)+c^3(a+b)]-6[(ab+ac+bc)^2-2abc(a+b+c)] a 4 + b 4 + c 4 = ( 6 ) 4 12 ( 6 ) ( 6 ) 4 [ a 3 ( 6 a ) + b 3 ( 6 b ) + c 3 ( 6 c ) ] 6 [ ( 11 ) 2 2 ( 6 ) ( 6 ) ] a^4+b^4+c^4=(6)^4-12(6)(6)-4[a^3(6-a)+b^3(6-b)+c^3(6-c)]-6[(11)^2-2(6)(6)] a 4 + b 4 + c 4 = 1296 432 4 [ ( 6 a 3 a 4 + 6 b 3 b 4 + 6 c 3 c 4 ) ] 294 = a^4+b^4+c^4=1296-432-4[(6a^3-a^4+6b^3-b^4+6c^3-c^4)]-294= a 4 + b 4 + c 4 = 1296 432 4 [ 6 ( a 3 + b 3 + c 3 ) ( a 4 + b 4 + c 4 ) ] 294 = a^4+b^4+c^4=1296-432-4[6(a^3+b^3+c^3)-(a^4+b^4+c^4)]-294= a 4 + b 4 + c 4 = 1296 432 4 [ 6 ( 36 ) ( a 4 + b 4 + c 4 ) ] 294 = a^4+b^4+c^4=1296-432-4[6(36)-(a^4+b^4+c^4)]-294= a 4 + b 4 + c 4 = 1296 432 864 + 4 ( a 4 + b 4 + c 4 ) 294 = a^4+b^4+c^4=1296-432-864+4(a^4+b^4+c^4)-294= a 4 + b 4 + c 4 = 4 ( a 4 + b 4 + c 4 ) 294 = a^4+b^4+c^4=4(a^4+b^4+c^4)-294= ( a 4 + b 4 + c 4 ) = 294 3 = 98 \rightarrow\color{#D61F06}{(a^4+b^4+c^4)}=\frac{294}{3}=\Large\color{#3D99F6}{\boxed{98}}

Complicated

Varun Luthra - 5 years, 3 months ago

awesome work

Nikkil V - 5 years, 3 months ago

Cool, but it's messy. Maybe Newton's sums would be a better option

Hung Woei Neoh - 5 years, 1 month ago

did exactly the same way

abhishek alva - 3 years, 11 months ago
AAshish Shukla
Feb 12, 2016

It can be guessed pretty easily

Hung Woei Neoh
Apr 16, 2016

I prefer to use Vieta's formula and Newton's sums to do this.

From Vieta's formula, we have:

a + b + c = 6 a + b + c = 6

a b + a c + b c = x ab + ac + bc = x

a b c = y abc = y

To find a 4 + b 4 + c 4 a^4+b^4+c^4 , we will need to find x x and y y first.

Using Newton's sums:

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

14 = 6 ( 6 ) 2 x x = 11 a b + a c + b c = 11 14 = 6(6) - 2x\implies x=11\implies ab+ac+bc=11

Next,

a 3 + b 3 + c 3 = ( a + b + c ) ( a 2 + b 2 + c 2 ) ( a b + a c + b c ) ( a + b + c ) 3 a b c a^3+b^3+c^3 = (a+b+c)(a^2+b^2+c^2) - (ab+ac+bc)(a+b+c) - 3abc

36 = 6 ( 14 ) 11 ( 6 ) 3 y y = 6 a b c = 6 36 = 6(14) - 11(6) -3y\implies y=-6 \implies abc=-6

Then, substitute in here to find the answer:

a 4 + b 4 + c 4 = ( a + b + c ) ( a 3 + b 3 + c 3 ) ( a b + a c + b c ) ( a 2 + b 2 + c 2 ) a b c ( a + b + c ) = 6 ( 36 ) 11 ( 14 ) ( 6 ) ( 6 ) = 98 a^4+b^4+c^4 = (a+b+c)(a^3+b^3+c^3) - (ab+ac+bc)(a^2+b^2+c^2) -abc(a+b+c)\\ =6(36) - 11(14) -(-6)(6) = \boxed{98}

Note: Is this really a level 1 Algebra question?

I think that there is a slight mistake in the solution presented although the result is correct. It should be y=6, because the factorization of the terms a^3+b^3+c^3 should end with +3ab. The same mistake is in the next factorization so the error cancels out

Hermann Set - 2 years, 7 months ago

1+2+3 = 6 and they cannot be the same number as 2+2+2 so I used this method 1+2+3 and squared it , cubed it and I did it to the power of 4

BTW i am only 10 yrs old

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