x 6 3 + x 4 4 + x 3 7 + x 3 1 + x 2 6 + x 9 + 6
If x satisfies the equation ( x + x 1 ) 2 = 3 , then find the value of the expression above.
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Nice solution. Another way I did is ::
x 2 + x 2 1 + 2 = 3
⟹ x 2 + x 2 1 = 1
⟹ x 2 = − ω o r x 2 = − ω 2 ω being a cube root of unity.
So using the properties of ω , Every term of the given expression is cancelled out except 6 . S0 6 is the value of the given expression. @Sanjeet Raria
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done the same way.
Done the same way
even i did it in the same way :)
In response to Sanjeet Raira: can you explain the step how x^3+1/x^3=0 from previous step?
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x 2 + x 2 1 = 1
x 2 + x 2 1 − 1 = 0
( x + x 1 ) ( x 2 + x 2 1 − 1 ) = 0
x 3 + x 3 1 = 0
This will help us,
take the power 37 and 31 , 2 3 7 + 3 1 = 3 4
x 3 7 + x 3 1 = x 3 4 ( x 3 + x 3 1 )
now take power 44 and 26, 2 4 4 + 2 6 = 3 5
x 4 4 + x 2 6 = x 3 5 ( x 9 + x 9 1 ) (apply a 3 + b 3 )
take 63 and 9 , see a pattern of power continues , first it was 34 then 35 so we can take 36 , or once again we can apply the same procedure to find which power to be taken common, 2 6 3 + 9 = 3 6
or x 3 n + x 3 n 1 = 0 (as x 3 + x 3 1 = 0 )
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Thanks for your answer @Megh Choksi, why are you multiplying (x+1/x)?
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@Sridhar Muruganandham – For making x 3 + x 3 1 , sanjeet raria sir cubed both the sides to get it zero
( x + x 1 ) : A form you like. @brian charlesworth
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Nice problem and solution, Sanjeet. My approach was the same as Sujoy's, once I observed that x 4 − x 2 + 1 was a factor x 6 + 1 .
I ALSO DID LIKE THIS!!
I agree that x^3 + 1/x^3 is 0, however, would not x^6 + 1/x^6 = -2? That is at least what I get when squaring the first expression x^3 + 1/x^3
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i am also confused about that
but i think the expression should be like this
when x 3 + x 3 1 = 0 then x 3 n + x 3 n 1 = 0 too, provided n ∈ N }
which applies to only x 3 + x 3 1 , x 9 + x 9 1 , x 2 7 + x 2 7 1 and so on.
using the similar calculation to form x 3 + x 3 1 from x + x 1 and x 2 − 1 + x 2 1 , we could have :
( x 3 + x 3 1 ) ⋅ ( x 6 − 1 + x 6 1 ) = x 9 + x 9 1
hence x 9 + x 9 1 = 0 too
x 2 7 + x 2 7 1 and higher would have a similar way
correct me if there were mistakes
x^3+1/x^3 = 0 suppose for n<=k, x^3n+1/x^3n = 0 now, (x^3k+1/x^3k)(x^3+1/x^3) = 0 = x^3(k+1)+x^3(k-1)+1/x^3(k-1)+1/x^3(k+1) now, x^3(k-1)+1/x^3(k-1) = 0 by our inductive step so, x^3(k+1)+1/x^3(k+1) = 0 this proves the induction
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What about x^6+1/x^6, x^12+1/x^12.....????
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x^6+1/x^6 = 0 because of my proof above same for x^12+1/x^12...
do you intentionally put the answer "6.00" to confuse members (liked this)?
same method!
Slight mistake in your rule x^(3n)+x^(-3n)=0 this works for n=3,9 but doesn't work for n=2, which equals -2. Not sure what the exact rule is for which n the identity works for but it does not work for all natural n.
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Exactly.. This mistake also puzzled me for few minutes...
Nice Problem ! :D
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Thank you.
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I am getting a different answer by Wolfram Alpha. Please see a comment below.
x^3n+1/x^3n=0 for all natural number is incorrect when x^6+1/x^6, x^12+1/x^12....etc. it would be correct only 3n=3,9,27,81,.... Please rectify me if I may be wrong.
Exactly how I solved it!
Wonderful Solution!
how did you do the 3rd step in the first line
Nice solution sir..................really a elegant one............But sir i was having a doubt in buying resnick,halliday and walker book............actually there are so many editions coming that i am a bit confused which one to buy even resnick,halliday and krane is also coming ...............sir please help and if possible send me a link......i would be thankful to you...........
( x + 1 / x ) 2 = 3 , ⟹ ( x + 1 / x ) 3 = 3 3 . ∴ 3 3 = ( x + 1 / x ) 3 = ( x 3 + 1 / x 3 ) + 3 ∗ x ∗ 1 / x ∗ ( x + 1 / x ) = x 3 + 1 / x 3 + 3 3 . ⟹ x 3 + 1 / x 3 = 0 . L e t x = y 3 , ∴ y 3 + 1 / y 3 = 0 . ⟹ , x 9 + 1 / x 9 = 0 . S i m i l a r l y , x 2 7 + 1 / x 2 7 = 0 x 3 n + 1 / x 3 n = 0 f o r e v e r y n ∈ N . x 3 n + 1 / x 3 n = 0 f o r e v e r y n ∈ N .
( x 2 + x 2 1 ) 2 = 3
or, x 4 − x 2 + 1 = 0
or, ( x 2 + 1 ) ( x 4 − x 2 + 1 ) = 0
or, x 6 + 1 = 0 or, x 6 = − 1
So, x 6 3 + x 4 4 + x 3 7 + x 3 1 + x 2 6 + x 9 + 6
= x 3 − x 2 + x − x + x 2 − x 3 + 6 = 6
Used complex number(De- Moivre's theorem) to solve, Method of solving is quite similar.
Nice solution.
Awesome. I aappreciate your solution. I really liked the way you did this.
good solution simple but not complicated
same method (Y)
I have up voted as the best and simple direct solution.
Recall that cos θ = 2 e i θ + e − i θ . Substituting in x = e i θ gives
cos θ = 2 3
Since the ratio of the adjacent side to the hypotenuse in a 30-60-90 triangle from the angle π / 6 is 2 3 , we can conclude
x = e i θ = e i π / 6 = ( − 1 ) 1 / 6
It can be easily seen that x 6 m + n = ( − 1 ) m x n . Hence, our given expression is equal to
x 3 − x 2 + x − x + x 2 − x 3 + 6 = 6
( x + x 1 ) 2 = 3
= > x 2 + x 2 1 = 1
= > x n = x n − 2 − x n − 4 by multiplying x n − 2 on both sides
Hence, the above also yields = > x n − 2 = x n − 4 − x n − 6 , where substituting it back in to the previous equation yields
x n = − x n − 6
Using this, the expression :
x 6 3 + x 4 4 + x 3 7 + x 3 1 + x 2 6 + x 9 + 6
= x 3 − x 2 + x − x + x 2 − x 3 + 6
= 6
Obviously, From the problem, we have: ( x + x 1 ) 2 = x 2 + x 2 1 + 2 = 3 ⇒ x 2 + x 2 1 = 1
Therefore: x 3 + x 3 1 = ( x + x 1 ) ( x 2 + x 2 1 − 1 ) = ( x + x 1 ) ⋅ 0 = 0 . Furthermore, we can prove by induction:
( x ( 3 n ) + x ( 3 n ) 1 ) = 0 , when ( x + x 1 ) 2 = 3 and n ∈ N
Now, our expression is: x 6 3 + x 4 4 + x 3 7 + x 3 1 + x 2 6 + x 9 + 6 = x 3 5 ( x 9 + x 9 1 ) + x 3 6 ( x 2 7 + x 2 7 1 ) + x 3 4 ( x 3 + x 3 1 ) + 6 = 0 + 0 + 0 + 6 = 6
Good observation with the inductive step.
Do you know how to state the value of x in a simple manner?
On mobile i can't see last digit 6,so i made error,please shorten it. Display smaller letters to fit on any screen.
This will be easy if we put this on the complex plane.
( x + x 1 ) 2 = 3
x 2 + 2 + x 2 1 = 3
x 4 − x 2 + 1 = 0
The roots will be c i s ( 6 Π ), c i s ( 6 5 Π ), c i s ( 6 7 Π ), and c i s ( 6 1 1 Π ).
Then substitute to the expression, (let's use c i s ( 6 Π ))
x 6 3 + x 4 4 + x 3 7 + x 3 1 + x 2 6 + x 9 + 6
= c i s ( 6 3 × 6 Π ) + c i s ( 4 4 × 6 Π ) + c i s ( 3 7 × 6 Π ) + c i s ( 3 1 × 6 Π ) + c i s ( 2 6 × 6 Π ) + c i s ( 9 × 6 Π ) + 6
= c i s ( 2 2 1 Π ) + c i s ( 3 2 2 Π ) + c i s ( 6 3 7 Π ) + c i s ( 6 3 1 Π ) + c i s ( 3 1 3 Π ) + c i s ( 2 3 Π ) + 6
= c i s ( 2 1 Π ) + c i s ( 3 − 2 Π ) + c i s ( 6 1 Π ) + c i s ( 6 − 1 Π ) + c i s ( 3 1 Π ) + c i s ( 2 − 1 Π ) + 6
= [ c o s ( 2 1 Π ) + c o s ( 3 − 2 Π ) + c o s ( 6 1 Π ) + c o s ( 6 − 1 Π ) + c o s ( 3 1 Π ) + c o s ( 2 − 1 Π ) + 6 ] + ι [ s i n ( 2 1 Π ) + s i n ( 3 − 2 Π ) + s i n ( 6 1 Π ) + s i n ( 6 − 1 Π ) + s i n ( 3 1 Π ) + s i n ( 2 − 1 Π ) ]
= 0
Using the other roots will also lead to 0 .
x 2 + x 2 1 = 1 . ⟹ x 4 = x 2 − 1 . . . . . ( A ) . ∴ x 8 = x 4 − 2 x 2 + 1 = − x 2 . . . . . ( B ) . ∴ x 6 = − 1 . . . . . . ( C ) . ⟹ x 1 2 = 1 . . . . ( D ) , a n d x 3 = i . . . . . . ( E ) B y ( D ) a n d ( E ) x 6 3 = ( x 1 2 ) 5 ∗ x 3 = i B y ( D ) a n d ( E ) x 4 4 = ( x 1 2 ) 3 ∗ x 8 = − x 2 B y ( D ) x 3 7 = ( x 1 2 ) 3 ∗ x = x B y ( D ) a n d ( C ) x 3 1 = ( x 1 2 ) 2 ∗ x 6 x = − x ( D ) x 2 6 = ( x 1 2 ) 2 ∗ x 2 = x 2 B y ( C ) a n d ( E ) x 9 = x 6 ∗ x 3 = − i ∴ x 6 3 + x 4 4 + x 3 7 + x 3 1 + x 2 6 + x 9 + 6 = i + − x 2 + x − x + x 2 − i + 6 = 6
Better still:
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evaluate x^63+x^44+x^37+x^31+x^26+x^9+6, (x+1/x)^2=3 (wolframalpha substitution case)
x^63+x^44+x^37+x^31+x^26+x^9+6=6
Try to find roots take those comples roota in euler form....put it in equation
Solve to get x=iw,Substitute x=iw in eq to get 6 where w is cube root of unity.
Here we get for ( x + 1 / x ) 2 = 3 we get x 2 = - ω [where ω is the cube root of unity ]
By further Solving x 6 3 + x 4 4 + x 3 7 + x 3 1 + x 2 6 + x 9 + 6
And putting the values care fully we get all cut down except 6. Hence Ans = 6
x^12=1, so just reduce the exponents.
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We have, ( x + x 1 ) 2 = 3 ⇒ ( x + x 1 ) = √ 3 ⇒ ( x 3 + x 3 1 ) = 0
Further it can be seen that for every n ∈ N , ( x 3 n + x 3 n 1 ) = 0 ( W h y )
Now our required expression is, x 3 7 + x 3 1 + x 4 4 + x 2 6 + x 6 3 + x 9 + 6 = x 3 4 ( x 3 + x 3 1 ) + x 3 5 ( x 9 + x 9 1 ) + x 3 6 ( x 2 7 + x 2 7 1 ) + 6 = x 3 4 ( 0 ) + x 3 5 ( 0 ) + x 3 6 ( 0 ) + 6 = 6
Any other solutions are welcomed for the sake of variety.