If α and β are the roots of the equation x 2 + x + 1 = 0 , then the product of the roots of the equation whose roots are α 1 9 and β 7 is __________ .
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The roots of the equation x 2 + x + 1 = 0 are α = e i 2 π / 3 = cos 2 π /3 + i sin 2 π /3 and β = e i 4 π / 3 , and the product of the roots of the equation whose roots are α 1 9 and β 7 is α 1 9 β 7 = e i 3 8 π / 3 e i 2 8 π / 3 = e i 2 2 π = 1, Note : I remember e i α = cos α + i sin α
I don't understand how the argument of beta is 4Π/3. Couldn't it be Π/3??
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No, β 2 + β = e 3 8 i π + e 3 4 i π = = = ( cos 3 8 π + i sin 3 8 π ) + ( cos 3 4 π + i sin 3 4 π ) = − 1
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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.
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.
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
The roots of x 2 + x + 1 are imaginary because 2 − 1 ± 1 − 4 = 2 − 1 ± 3 i . Then we multiply it 19 times, and 7 times, then we should multiply ( 2 − 1 + 3 i ) 1 9 and ( 2 − 1 − 3 i ) 7 . Then ( 2 − 1 + 3 i ) 1 9 × ( 2 − 1 − 3 i ) 7 . So we get 1 .
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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Note first that ( x 2 + x + 1 ) ( x − 1 ) = x 3 − 1 , so α and β are the cubed roots of unity other than 1 .
By Vieta's we know that α β = 1 , so whichever root we assign as α and which as β we have that ( α β ) 7 = 1 . Also, both of these roots raised to the power of 1 2 equal 1 , as they are both cubed roots of unity. So however we assign α and β we have that
( α ) 1 9 ( β ) 7 = ( α β ) 7 ( α ) 1 2 = 1 .