But they aren't the same

Algebra Level 1

If α \alpha and β \beta are the roots of the equation x 2 + x + 1 = 0 , x^2 + x + 1 = 0, then the product of the roots of the equation whose roots are α 19 \alpha^{19} and β 7 \beta ^7 is __________ . \text{\_\_\_\_\_\_\_\_\_\_}.


The answer is 1.

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5 solutions

Note first that ( x 2 + x + 1 ) ( x 1 ) = x 3 1 (x^{2} + x + 1)(x - 1) = x^{3} - 1 , so α \alpha and β \beta are the cubed roots of unity other than 1. 1.

By Vieta's we know that α β = 1 \alpha\beta = 1 , so whichever root we assign as α \alpha and which as β \beta we have that ( α β ) 7 = 1. (\alpha\beta)^{7} = 1. Also, both of these roots raised to the power of 12 12 equal 1 1 , as they are both cubed roots of unity. So however we assign α \alpha and β \beta we have that

( α ) 19 ( β ) 7 = ( α β ) 7 ( α ) 12 = 1 . (\alpha)^{19}(\beta)^{7} = (\alpha\beta)^{7}(\alpha)^{12} = \boxed{1}.

The roots of the equation x 2 + x + 1 = 0 x^2 + x + 1 = 0 are α \alpha = e i 2 π / 3 e^{i2\pi/3} = cos 2 π \pi /3 + i sin 2 π \pi /3 and β \beta = e i 4 π / 3 e^{i4\pi/3} , and the product of the roots of the equation whose roots are α 19 \alpha^{19} and β 7 \beta^{7} is α 19 \alpha^{19} β 7 \beta^{7} = e i 38 π / 3 e^{i38\pi/3} e i 28 π / 3 e^{i28\pi/3} = e i 22 π e^{i22\pi} = 1, Note : I remember e i α e^{i\alpha} = cos α \alpha + i sin α \alpha

I don't understand how the argument of beta is 4Π/3. Couldn't it be Π/3??

Puneet Pinku - 5 years ago

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No, β 2 + β = e 8 i π 3 + e 4 i π 3 = \beta^2 + \beta = e^{\frac{8i\pi}{3}} + e^{\frac{4i\pi}{3}} = = = ( cos 8 π 3 + i sin 8 π 3 ) + ( cos 4 π 3 + i sin 4 π 3 ) = 1 = (\cos \frac{8\pi}{3} + i \sin \frac{8\pi}{3}) + (\cos \frac{4\pi}{3} + i \sin \frac{4\pi}{3}) = - 1

Guillermo Templado - 5 years ago

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Which means alpha +beta =-1. Okay, thanks. I found another way to find the angle. We must see the signs of z and try to plot it in the graph, then which is the right argument is easily found. I was confused after taking the arctan of b/a. I got √3 as my value which is tangent of both Π/3 and4Π/3. Later I saw that the signs in the complex number indicate its presence in 3rd quadrant which corresponds to the argument 4Π/3.

Puneet Pinku - 5 years ago

Beta is (-1/2) + i*(-√(3)/2), which indicates that both the sine and cosine are negative values. This is in the third quadrant. Thus, we have the argument of Beta as 4π/3, which is in third quadrant rather than π/3, which is in first quadrant.

Madhav Karnani - 4 months, 1 week ago
Kunal Sahni
Jul 12, 2016

W= omega and w^2 are roots of the given eqn . And they ask for w^19 multiplied with (w^2)^7 . Giving w^33 . Using w^3=1 we finally get the answer 1

. .
Feb 1, 2021

The roots of x 2 + x + 1 x^{2}+x+1 are imaginary because 1 ± 1 4 2 \frac{-1 \pm \sqrt{1-4}}{2} = 1 ± 3 i 2 \frac{-1 \pm \sqrt{3}i}{2} . Then we multiply it 19 times, and 7 times, then we should multiply ( 1 + 3 i 2 ) 19 (\frac{-1 + \sqrt{3}i}{2})^{19} and ( 1 3 i 2 ) 7 (\frac{-1 - \sqrt{3}i}{2})^{7} . Then ( 1 + 3 i 2 ) 19 × ( 1 3 i 2 ) 7 (\frac{-1 + \sqrt{3}i}{2})^{19} \times (\frac{-1 - \sqrt{3}i}{2})^7 . So we get 1 1 .

Art Vocals
Jan 15, 2021

x = a or x = b x-a = 0 or x-b = 0 (x-a)(x-b) = 0 by multiplying x^2 - (a+b)x + a b = 0 therefore we have our equation x^2 + x + 1 =0 by comparing with the GENERAL equation !!! a b = 1 and as we 1 to the nth power is 1 therefore a^19 * b^7 =1

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