Can you manipulate this?

Algebra Level 2

If x 2 x 1 = 0 x^2-x-1 =0 , then find the value of x 3 2 x + 1 x^3-2x+1 .


The answer is 2.

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6 solutions

Chew-Seong Cheong
Aug 31, 2016

It is given that x 2 x 1 = 0 x^2 - x - 1 = 0 x 2 = x + 1 \implies \color{#3D99F6}{x^2 = x+1} x 3 = x 2 + x = ( x + 1 ) + x = 2 x + 1 \implies \color{#D61F06}{x^3} = \color{#3D99F6}{x^2} + x = \color{#3D99F6}{(x+1)} + x \color{#D61F06}{= 2x+1} .

Therefore, x 3 2 x + 1 = ( 2 x + 1 ) 2 x + 1 = 2 \color{#D61F06}{x^3}-2x+1 = \color{#D61F06}{(2x+1)} -2x +1 = \boxed{2} .

Sir, I suppose there wouldn't be a square sign in the last line while replacing for x^3..

Puneet Pinku - 4 years, 9 months ago

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Yes, thanks.

Chew-Seong Cheong - 4 years, 9 months ago

Could you also please solve my problem on the link

Puneet Pinku - 4 years, 9 months ago

Sir, please me with the problem on this link. https://brilliant.org/discussions/thread/algebra-mania/

Puneet Pinku - 4 years, 9 months ago
Yatin Khanna
Aug 31, 2016

Let x 3 2 x + 1 = S x^3 - 2x+1=S
Then; x 3 + 1 = S + 2 x x^3+1=S+2x
( x + 1 ) ( x 2 x + 1 ) = S + 2 x \implies (x+1)(x^2-x+1)=S+2x
(Since, a 3 + b 3 = ( a + b ) ( a 2 a b + b 2 ) \color{#3D99F6}{a^3 + b^3 = (a+b)(a^2-ab+b^2)} )
2 ( x + 1 ) = S + 2 x \implies2(x+1) = S +2x
(Since, x 2 x 1 = 0 ; = = > x 2 x + 1 = 2 \color{#3D99F6}{x^2-x-1=0; ==> x^2-x+1=2} )
2 x + 2 = S + 2 x \implies 2x + 2 = S+2x
Hence, x 3 2 x + 1 = S = 2 x^3-2x+1=S=\boxed{2}



Try this question!! https://brilliant.org/discussions/thread/algebra-mania/

Puneet Pinku - 4 years, 9 months ago
Jihoon Kang
Aug 31, 2016

x 3 2 x + 1 = ( x 2 x 1 ) ( x + 1 ) + 2 x^3-2x+1 = (x^2-x-1)(x+1)+2

= ( 0 ) ( x + 1 ) + 2 = (0)(x+1)+2

= 2 =2

Please help me in solving this problem, https://brilliant.org/discussions/thread/algebra-mania/

Puneet Pinku - 4 years, 9 months ago
Puneet Pinku
Aug 31, 2016

The given expression can be be rearranged as

x 3 2 x 1 = x ( x 2 2 ) + 1 = x ( x + 1 2 ) + 1 x^3-2x-1 = x(x^2-2)+1= x(x+1-2)+1 as x 2 x 1 = 0 x 2 = x + 1 x^2-x-1 = 0 \Rightarrow x^2 = x+1

= x 2 x + 1 = x 2 x 1 + 2 = 2 = x^2-x +1 = x^2-x-1+2 = 2 as x 2 x 1 = 0 x^2-x-1= 0

Hence the answer is 2.

Daniel Ferreira
Nov 5, 2016

x 3 2 x + 1 = ( x 3 + 1 ) 2 x = [ ( x + 1 ) ( x 2 x 1 ) + 2 x + 2 ] 2 x = [ ( x + 1 ) 0 + 2 x + 2 ] 2 x = 0 + 2 x + 2 2 x = 2 \\ x^3 - 2x + 1 = \\\\ (x^3 + 1) - 2x = \\\\ \left [ (x + 1)(x^2 - x - 1) + 2x + 2 \right ] - 2x = \\\\ \left [ (x + 1) \cdot 0 + 2x + 2 \right ] - 2x = \\\\ 0 + 2x + 2 - 2x = \\\\ \boxed{2}

x ² x 1 = 0 x²-x-1=0 . . . . . . . . . . ( 1 ) ..........(1)

x ² = x + 1 x²=x+1

Multiply both side with x x

x ³ = x ² + x x³=x²+x . . . . . . . . . . ( 2 ) ..........(2)

Otherwise, from ( 1 ) (1) we get

x = x ² 1 x = x²-1

Adding x x on both sides we get

2 x = x ² + x 1 2x = x² + x -1 . . . . . . . . . . ( 3 ) ..........(3)

Now,

x ³ 2 x + 1 = ( 2 ) ( 3 ) + 1 x³ -2x+1 = (2) - (3) +1

= x ² + x ( x ² + x 1 ) + 1 = x² + x - (x² + x -1) +1

= x ² + x x ² x + 1 + 1 = x² +x - x² -x +1 +1

= 2 = \boxed{2}

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