If x + x 1 = 1 , then find the value of
x + x 4 + x 7 + x 1 0 + x 1 9 + x 2 2 + x 2 5 + x 3 1 + x 3 4 + x 4 0 .
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Superb Solution
the roots of the equation x + x 1 = 1 are − ω a n d − ω 2 where ω , ω 2 are cubic roots of unity note that ⎩ ⎪ ⎨ ⎪ ⎧ ω x = ω , x = 1 m o d 3 ω x = ω 2 , x = 2 m o d 3 ω x = 1 , x = 0 m o d 3 now , insert the value − ω + ( − ω ) 4 + ( − ω ) 7 + ( − ω ) 1 0 + ( − ω ) 1 9 + ( − ω ) 2 2 + ( − ω ) 2 5 + ( − ω ) 3 1 + ( − ω ) 3 4 + ( − ω ) 4 0 from the cases, we get that the expression is − ω + ω − ω + ω − ω + ω − ω − ω + ω + ω the positive cancel out the negatives and we are left with 0
x + x 1 = − 1
or, x 2 + x + 1 = 0
or, ( x + 1 ) ( x 2 + x + 1 ) = 0 or, x 3 + 1 = 0 or, x 3 = − 1 .
Now, 1 7 2 9 + x + x 4 + x 7 + x 1 0 + x 1 9 + x 2 2 + x 2 5 + x 3 1 + x 3 4 + x 4 0
= 1 7 2 9 + x − x + x − x + x − x + x + x − x − x = 1 7 2 9
Note that ( x + 1 ) ( x 2 + x + 1 ) = x 3 + 1 .
I believe what you are thinking of is ( x − 1 ) ( x 2 + x + 1 ) which gives us x 3 − 1 . However, we may no longer continue with the solution.
What we get is that the value is 1 7 2 9 + 1 0 ω , where ω is one of the complex cube roots of unity. This is equal to 1 7 2 4 ± 5 3 i .
The answer has been updated to 1724.
Given the changes to the question, I have updated the answer to 0. Can you update your solution accordingly? Thanks!
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if x + x 1 = 1 then x 2 − x + 1 = 0
( x + 1 ) ( x 2 − x + 1 ) is a factor of x ( 1 + x 3 + x 6 + x 9 + x 1 8 + x 2 1 + x 2 4 + x 3 0 + x 3 3 + x 3 9 )
( x ) ( x + 1 ) ( x 2 − x + 1 ) ( 1 + x 6 + x 1 8 + x 2 4 − x 2 7 + 2 x 3 0 − x 3 3 + x 3 6 )
but x 2 − x + 1 = 0
So ( x ) ( x + 1 ) ( 0 ) ( 1 + x 6 + x 1 8 + x 2 4 − x 2 7 + 2 x 3 0 − x 3 3 + x 3 6 ) = 0