Polynomials with difficulty......

Algebra Level 2

If a + b + c = 0 a+b+c=0 and a b c 0 abc \neq 0 , find the value of a 4 + b 4 + c 4 a 2 b 2 + b 2 c 2 + c 2 a 2 \frac{ { a }^{ 4 }+{ b }^{ 4 }+{ c }^{ 4 } }{ { a }^{ 2 }{ b }^{ 2 }+ b^2c^2+c^2a^2 }


The answer is 2.

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8 solutions

Anik Mandal
Aug 26, 2014

a + b + c a+b+c = 0 0

a + b a+b = c -c

a 2 + b 2 + 2 a b a^{2}+b^{2}+2ab = c 2 c^2

a 2 + b 2 c 2 a^{2}+b^{2}-c^{2} = 2 a b -2ab

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

a 4 + b 4 + c 4 + 2 a 2 b 2 2 b 2 c 2 2 c 2 a 2 a^{4}+b^{4}+c^{4}+2a^{2}b^{2}-2b^{2}c^{2}-2c^{2}a^{2} = 4 a 2 b 2 4a^{2}b^{2}

a 4 + b 4 + c 4 a^{4}+b^{4}+c^{4} = 2 a 2 b 2 + 2 b 2 c 2 + 2 c 2 a 2 2a^{2}b^{2}+2b^{2}c^{2}+2c^{2}a^{2}

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

Hence,the result of the given expression is 2.

@Anik Mandal I like your solution nice !!!

Mardokay Mosazghi - 6 years, 9 months ago

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Then please upvote it.....I haven't had a single upvote since the last two weeks!So please... @Mardokay Mosazghi

Anik Mandal - 6 years, 9 months ago

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sure i up voted it you welcome

Mardokay Mosazghi - 6 years, 9 months ago

nyce approach bro

Saikarthik Bathula - 6 years, 9 months ago

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Thanks!!:D

Anik Mandal - 6 years, 9 months ago

Knowing the actual source,

With S = a + b + c, P = a b c and I = 1/ a + 1/ b + 1/ c,

a^4 + b^4 + c^4 = S^4 - 4 P I S^2 + 4 P S + 2 P^2 I^2

S = a + b + c = 0:

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

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

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

= 2

Lu Chee Ket - 6 years, 9 months ago
Ronak Agarwal
Aug 15, 2014

a + b + c = 0 a+b+c=0 Squaring both sides we get :

a 2 + b 2 + c 2 + 2 a b + 2 b c + 2 a c = 0 { a }^{ 2 }+{ b }^{ 2 }+{ c }^{ 2 }+2ab+2bc+2ac=0

Rearranging we get :

a 2 + b 2 + c 2 = 2 ( a b + b c + a c ) { a }^{ 2 }+{ b }^{ 2 }+{ c }^{ 2 }=-2(ab+bc+ac) again squaring both sides we get :

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

Using a + b + c = 0 a+b+c=0 and rearranging some terms we get :

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

Finally a 4 + b 4 + c 4 a 2 b 2 + a 2 c 2 + b 2 c 2 = 2 \boxed { \frac { { a }^{ 4 }+{ b }^{ 4 }+{ c }^{ 4 } }{ { a }^{ 2 }{ b }^{ 2 }+{ a }^{ 2 }{ c }^{ 2 }+{ b }^{ 2 }{ c }^{ 2 } } =2 }

But if you take values all as 0, then the answer is 0. The answer to this question is 0 not 2.

Anuj Shikarkhane - 6 years, 9 months ago

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No no no no no no the denominator will become 0. No no no no no no no

Samuraiwarm Tsunayoshi - 6 years, 9 months ago

I agree with your argument but in these types of questions you have to solve them using variables instead of no.s just like Ronak Agarwal has done

However ther is even a smarter way to solve it

Satyajit Ghosh - 6 years, 9 months ago

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But you should have mentioned that a,b,c are not equal to 1.

Anuj Shikarkhane - 6 years, 9 months ago

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@Anuj Shikarkhane If you take a=b=c=0, Then your answer will come as 0/0. Remember 0/0 is undefined so the answer can come as 1,2,3,....... This happens as 0* any no is 0. So we can put any no. in the answer if there is 0/0.

Satyajit Ghosh - 6 years, 9 months ago
Satyajit Ghosh
Dec 26, 2014

People I have been getting many notifications on clarification of the case if we take 0. I would like to tell you if we take a,b and c as 0, the situation comes out to be 0/0. Remember 0/0 is not defined as any no. divided by 0 is undefined so 0/0 can be any answer so in these questions we don't take it as 0/0 as answer. By the way 0/0 can also be the answer too. I hope I have cleared your doubt but do comment if it still persists and the way Anik Mandal solved is just the way you do it. Do share your different method of solving it:)

its simple for this problem does'nt need to think big take 3 values satisfying a+b+c=0 as a=2 b=-1 c=-1 thats it we get solution easily

Vinit Béléy
Sep 26, 2014

a + b + c = 0

(a + b + c)^2 = 0

a^2 + b^2 + c^2 + 2(ab + bc + ac) = 0

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

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

a^4 + b^4 + c^4 + 2[a^2.b^2 + b^2.c^2 + a^2.c^2] = 4[a^2.b^2 + b^2.c^2 + a^2.c^2 + 2abc (a + b + c ) ]

But a + b + c = 0 a^4 + b^4 + c^4 + 2[a^2.b^2 + b^2.c^2 + a^2.c^2] = 4[a^2.b^2 + b^2.c^2 + a^2.c^2]

a^4 + b^4 + c^4 = 2[a^2.b^2 + b^2.c^2 + a^2.c^2]

Hence, a^4 + b^4 + c^4 / [a^2.b^2 + b^2.c^2 + a^2.c^2] = 2

Ninad Akolekar
Sep 7, 2014

Put any three numbers a,b,c such that a+b+c = 0, and you'll get same answer in each case!! That's because its actually an identity in a,b,c for all real a+b+c = 0. This method is meant only to get quick answers and is not actual solution to the problem.

Christian Daang
Aug 30, 2014

a+b+c = 0 --> a+b = -c -->(a+b)^4 = (-c)^4 -->a^4 + (4a^3)b + (6a^2)(b^2) + 4ab^3 + b^4 = c^4

a^4 + b^4 + c^4 --> a^4 + b^4 + a^4 + (4a^3)b + (6a^2)(b^2) + 4ab^3 + b^4 = 2a^4 + (4a^3)b + (6a^2)(b^2) + 4ab^3 + 2b^4

(a+b)^2 = (-c)^2 -->(a^2 + 2ab + b^2) = c^2 -->(a^2)(b^2) + (b^2)(c^2) + (c^2)(a^2) --> (a^2)(b^2) + (a^2 + 2ab + b^2)b^2 + a^2(a^2 + 2ab + b^2) = a^4 + (2a^3)b + (3a^2)(b^2) + 2ab^3 + b^4

So, there fore... (2a^4 + (4a^3)b + (6a^2)(b^2) + 4ab^3 + 2b^4)/(a^4 + (2a^3)b + (3a^2)(b^2) + 2ab^3 + b^4)

= 2 (Q.E.D)

Jaiveer Shekhawat
Aug 18, 2014

the solution given by ronak agarwal is often used... if u don't follow his method, you can take a=1 b=-1 and c=0 or any other solution set where a+b+c=0 then also u will get "2"

I used exact same example. ..

Rajsuryan Singh - 6 years, 9 months ago

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