Bash me up please!

Algebra Level 4

{ a + b + c = 1 a 2 + b 2 + c 2 = 2 a 3 + b 3 + c 3 = 3 \large\large{ \begin{cases} a+b+c=1 \\ a^2+b^2+c^2 = 2 \\ a^3 + b^3+c^3 = 3 \\ \end{cases} }

Find a 5 + b 5 + c 5 \large\large a^5 + b^5 + c^5 .

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The answer is 6.

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2 solutions

Sam Bealing
Jun 19, 2016

p = a + b + c = 1 q = a b + b c + a c r = a b c p=a+b+c=1 \\ q=ab+bc+ac \\ r=abc

p 2 = 1 = a 2 + 2 ( a b + b c + a c ) 2 + 2 q = 1 q = 1 2 p^2=1=\sum a^2+2(ab+bc+ac) \implies 2+2q=1 \implies q=-\dfrac{1}{2}

f ( x ) = ( x a ) ( x b ) ( x c ) = x 3 p x 2 + q x r = x 3 x 2 x 2 r f ( a ) = f ( b ) = f ( c ) = 0 f(x)=(x-a)(x-b)(x-c)=x^3-px^2+qx-r=x^3-x^2-\dfrac{x}{2}-r \quad \quad f(a)=f(b)=f(c)=0

f ( a ) = 0 a 3 a 2 a 2 r = 0 a 3 = a 2 + a 2 + r a 3 = 3 = a 2 + a 2 + 3 r = 5 2 + 3 r 3 r = 1 2 r = 1 6 f(a)=0 \implies a^3-a^2-\dfrac{a}{2}-r=0 \implies a^3=a^2+\dfrac{a}{2}+r \\ \sum a^3=3=\sum a^2+\dfrac{\sum a}{2}+3r=\dfrac{5}{2}+3r \implies 3r=\dfrac{1}{2} \implies r=\dfrac{1}{6}

a 3 = a 2 + a 2 + 1 6 a 4 = a 3 + a 2 2 + a 6 a 4 = a 3 + a 2 2 + a 6 = 3 + 2 2 + 1 6 = 25 6 a^3=a^2+\dfrac{a}{2}+\dfrac{1}{6} \implies a^4=a^3+\dfrac{a^2}{2}+\dfrac{a}{6} \\ \sum a^4=\sum a^3+\dfrac{\sum a^2}{2}+\dfrac{\sum{a}}{6}=3+\dfrac{2}{2}+\dfrac{1}{6}=\dfrac{25}{6}

a 3 = a 2 + a 2 + 1 6 a 5 = a 4 + a 3 2 + a 2 6 a 5 = a 4 + a 3 2 + a 2 6 = 25 6 + 3 2 + 2 6 = 36 6 = 6 a^3=a^2+\dfrac{a}{2}+\dfrac{1}{6} \implies a^5=a^4+\dfrac{a^3}{2}+\dfrac{a^2}{6} \\ \sum a^5=\sum a^4+\dfrac{\sum a^3}{2}+\dfrac{\sum a^2}{6}=\dfrac{25}{6}+\dfrac{3}{2}+\dfrac{2}{6}=\dfrac{36}{6}=6

a 5 + b 5 + c 5 = 6 a^5+b^5+c^5=\color{#20A900}{\boxed{\boxed{6}}}

Moderator note:

Note: It is easier to state that you are using Newton's identities, then (essentially) proving them.

Really a very nice and elegant approach.................. +1 upvote

Abhisek Mohanty - 4 years, 11 months ago

Using Newton sum.

We get,
a b + b c + c a = 1 2 \Rightarrow ab+bc+ca=-\dfrac{1}{2}

a b c = 1 6 \Rightarrow abc=\dfrac{1}{6}

a 4 + b 4 + c 4 = 25 6 \Rightarrow a^4+b^4+c^4=\dfrac{25}{6}

And,

a 5 + b 5 + c 5 = ( a + b + c ) ( a 4 + b 4 + c 4 ) ( a b + b c + c a ) ( a 3 + b 3 + c 3 ) + a b c ( a 2 + b 2 + c 2 ) a^5+b^5+c^5=(a+b+c)(a^4+b^4+c^4)-(ab+bc+ca)(a^3+b^3+c^3)+abc(a^2+b^2+c^2)

a 5 + b 5 + c 5 = 25 6 + 3 2 + 1 3 = 36 6 a^5+b^5+c^5=\dfrac{25}{6}+\dfrac{3}{2}+\dfrac{1}{3}=\dfrac{36}{6}

a 5 + b 5 + c 5 = 6 \therefore a^5+b^5+c^5=\boxed{6}

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