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Algebra Level 3

If a , b , c a,b,c are distinct roots of the equation x 3 1 = 0 x^3-1=0 , find the value of a 1729 + b 1729 + c 1729 a^{1729}+b^{1729}+c^{1729} .


The answer is 0.0.

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3 solutions

Nihar Mahajan
Mar 6, 2016

Note that ( a , b , c ) = ( 1 , ω , ω 2 ) (a,b,c)=(1,\omega,\omega^2) where ω \omega is complex cube root of unity. Hence ,

a 1729 + b 1729 + c 1729 = 1 1729 + ω 1729 + ω 3458 a^{1729}+b^{1729}+c^{1729}=1^{1729}+\omega^{1729}+\omega^{3458}

Now using ω 3 = 1 \omega^3=1 , we have:

1 1729 + ω 1729 + ω 3458 = 1 + ω + ω 2 = 0 1^{1729}+\omega^{1729}+\omega^{3458}=1+\omega+\omega^2=\boxed{0}

Moderator note:

Good approach making use of x 3 = 1 x^3 = 1 .

Did it the same way... #cuberootofunityrocks :>

abc xyz - 5 years, 3 months ago
Rishabh Jain
Mar 6, 2016

If you don't have an idea of cube roots of unity, here's an alternate approach. x 3 = 1 ( x 3 ) 576 = 1 576 = 1 x 1728 = 1 x 1729 = x x^3=1\\ ~~~~~~~~\implies (x^3)^{576}=1^{576}=1\\ \implies x^{1728}=1\\ \implies x^{1729}=x a 1729 = a , b 1729 = b , c 1729 = c \implies a^{1729}=a, b^{1729}=b, c^{1729}=c a 1729 + b 1729 + c 1729 = a + b + c = 0 \therefore a^{1729}+b^{1729}+c^{1729} \\ =a+b+c=0 Since sum of roots of x 3 1 = 0 x^3-1=0 is 0 1 = 0 \frac{0}{1}=0 .

a 1729 + b 1729 + c 1729 = a 1728 a + b 1728 b + c 1728 c a^{1729}+b^{1729}+c^{1729} = a^{1728}\cdot a+b^{1728}\cdot b+c^{1728}\cdot c ( a 3 ) 576 a + ( b 3 ) 576 b + ( c 3 ) 576 c = a + b + c (a^{3})^{576}\cdot a + (b^{3})^{576}\cdot b+(c^{3})^{576}\cdot c=a+b+c Using Vieta's Formula we have a + b + c = 0 a+b+c=0

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