A Sum of Roots of Roots

Algebra Level 3

The real number x x satisfying the equation x + 2 x 1 + x 2 x 1 = 10 \sqrt{ x + \sqrt{2x-1} } + \sqrt{x - \sqrt{2x-1} } = 10 can be written as a b \frac{a}{b} where a a and b b are positive coprime integers. Find a + b a+b .


The answer is 53.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

22 solutions

Jan J.
Aug 5, 2013

Square both sides to get $$x + 2\sqrt{\left(x + \sqrt{2x - 1}\right)\left(x - \sqrt{2x - 1}\right)} + x = 100$$ $$2x + 2\sqrt{x^2 - 2x + 1} = 100$$ $$x + \sqrt{(x - 1)^2} = 50$$ $$x + |x - 1| = 50$$ If x 1 0 x - 1 \leq 0 , then we get 1 = 50 1 = 50 , so no solution.

If x 1 0 x - 1 \geq 0 , then we get $$x = \frac{51}{2}$$ Hence $$a + b = 51 + 2 = \boxed{53}$$

This is basically second problem from IMO 1959.

Moderator note:

Indeed, that was the inspiration behind this problem.

As you can see from the other solutions, almost everyone claims that 4 x 2 = 1 0 2 4x-2 = 10^2 , which arises from the incorrect claim that x 2 2 x + 1 = x 1 \sqrt{x^2 - 2x + 1 } = x-1 .

where did the x came from? the 1st and the last..

Crimson Jcp - 7 years, 10 months ago
Armin Namavari
Aug 4, 2013

Squaring both sides gives x + 2 x 1 + x 2 x 1 + 2 x 2 2 x + 1 = 100 x + \sqrt{2x - 1} + x - \sqrt{2x - 1} + 2\sqrt{x^2 - 2x + 1} = 100 which can be simplified to 2 x + 2 ( x 1 ) 2 = 100 2x + 2\sqrt{(x - 1)^2} = 100 which can further be simplified to 4 x 2 = 100 4x - 2 = 100 solving for x we get 51 2 \dfrac{51}{2} , 51 + 2 = 53, our final answer.

You forgot that ( x 1 ) 2 = x 1 \sqrt{(x - 1)^2} = |x - 1| .

Jan J. - 7 years, 10 months ago
Jason Tang
Aug 5, 2013

To get rid of the nested radicals, let's both sides of the equation. This gives us x + 2 x 1 + x 2 x 1 + 2 x 2 ( 2 x 1 ) = 100 x+\sqrt{2x-1}+x-\sqrt{2x-1}+2\sqrt{x^2-(2x-1)}=100 Cancelling out and factoring gives us 2 x + 2 ( x 1 ) 2 = 100 2x+2\sqrt{(x-1)^2}=100 2 x + 2 ( x 1 ) = 100 2x+2(x-1)=100 2 x 1 = 50 2x-1=50 2 x = 51 2x=51 x = 51 2 x=\frac{51}{2} 51 + 2 = 53 51+2=\boxed{53}

Zheng Wei
Aug 4, 2013

Let a=x, b= 2 x 1 \sqrt{2x-1}

a + b \sqrt{a+b} + a b \sqrt{a-b} =10

Square both sides

a+b+2 ( a + b ) ( a b ) \sqrt{(a+b)(a-b)} +a-b=100

Rearranging and divide by 2 gives

( a + b ) ( a b ) \sqrt{(a+b)(a-b)} =50-a

Squaring both sides gives :

a 2 a^{2} - b 2 b^{2} =2500-100a+ a 2 a^{2}

Substitute b 2 b^{2} =2x-1

And a=x

a 2 a^{2} -2a+1=2500-100a+ a 2 a^{2}

Rearranging gives:

98a=2499

a= 51 2 \frac{51}{2}

51+2=53

Faltu itne lambe jhamele mein fansa

Anshuman Singh - 7 years, 10 months ago

\sqrt{x + \sqrt{2x - 1}} + \sqrt{x - \sqrt{2x - 1}} = 10 \Rightarrow (\sqrt{x + \sqrt{2x - 1}} + \sqrt{x - \sqrt{2x - 1}}) ^ 2 = 100 \Rightarrow 2x + 2 \times (\sqrt{x + \sqrt{2x - 1}} \times \sqrt{x - \sqrt{2x - 1}}) = 100 \Rightarrow 2x+ 2 \times ( x - 1) = 100 \Rightarrow 4x = 102 \Rightarrow x = \frac {51}{2} Therefore a + b = 51 + 2 = 53

Hrishikesh Boro
Aug 5, 2013

x + 2 x 1 + x 2 x 1 = 10 \sqrt{x+\sqrt{2x-1}}+\sqrt{x-\sqrt{2x-1}}=10 = > ( x + 2 x 1 + x 2 x 1 ) 2 = 1 0 2 =>(\sqrt{x+\sqrt{2x-1}}+\sqrt{x-\sqrt{2x-1}})^2=10^{2} = > ( x + 2 x 1 ) 2 + 2 x + 2 x 1 x 2 x 1 + ( x 2 x 1 ) 2 = 100 =>(\sqrt{x+\sqrt{2x-1}})^2+2*\sqrt{x+\sqrt{2x-1}}*\sqrt{x-\sqrt{2x-1}}+(\sqrt{x-\sqrt{2x-1}})^{2}=100 = > x + 2 x 1 + 2 ( x + 2 x 1 ) ( x 2 x 1 ) + x 2 x 1 = 100 =>x+\sqrt{2x-1}+2*\sqrt{(x+\sqrt{2x-1})( {x-\sqrt{2x-1} )}}+x-\sqrt{2x-1}=100 = > 2 x + 2 x 2 ( 2 x 1 ) 2 = 100 =>2x+2*\sqrt{x^{2}-(\sqrt{2x-1})^{2}}=100 = > 2 x + x 2 2 x + 1 = 100 =>2x+\sqrt{x^{2}-2x+1}=100 = > 2 x + 2 ( x 1 ) = 100 =>2x+2*(x-1)=100 = > 2 x + 2 x 2 = 100 =>2x+2x-2=100 = > 4 x = 102 =>4x=102 = > x = 102 4 = 51 2 =>x=\frac{ 102 }{ 4 }=\frac{ 51 }{ 2 } Therefore,a=51,b=2 a+b=53

Liam Lawson
Aug 4, 2013

square both sides to get ( x + 2 x 1 ) + 2 ( x + 2 x 1 ) ( x s q r t 2 x 1 ) + ( x 2 x 1 ) = 100 (x + \sqrt{2x-1}) + 2\sqrt{(x + \sqrt{2x-1})(x-sqrt{2x-1})} + (x - \sqrt{2x-1}) = 100 the first and third terms can be added for simplification, and the second term can also be simplified using difference of two squares. the result is this: 2 x + 2 x 2 2 x + 1 = 100 2x + 2\sqrt{x^2-2x+1}=100 the second term involves the square root of a perfect square, so it becomes: 4 x 2 = 100 4x-2=100 this is easily solved as: x = 51 2 x=\frac{51}{2} so the solution is 51 + 2 = 53 51+2=53

Yash Kumar Gupta
Aug 10, 2013

lets (2x-1)=t^2 so we have x= (t^2+1)/2 => sqrt(t^2+1+2t)/2 + sqrt(t^2+1-2t)/2 = 10 => solving the Bracket => t+1 + t-1 = 10sqrt2 => t=5sqrt2 => t^2 = 50 => x = 51/2 = a/b so a+b= 53 :D

Naveen Kumar
Aug 8, 2013

squaring on both side we get

x+√2x+1 +x-√2x-1 +2√(x-1)2 =100

2x+2(x-1)=100

x+x-1=50

2x=51

x=51/2

where a=51 and b=2

so a+b= 51+2=53

Steven Yang
Aug 8, 2013

First off, we shift over one of the square roots.

x + 2 x 1 = 10 x 2 x 1 \sqrt{x + \sqrt{2x - 1}} = 10 - \sqrt{x - \sqrt{2x -1 }}

Squaring each side, we arrive at:

x + 2 x 1 = 100 20 x 2 x 1 + x 2 x 1 x + \sqrt{2x - 1} = 100 - 20\sqrt{x - \sqrt{2x - 1}} + x - \sqrt{2x - 1}

Continuing,

20 x 2 x 1 = 100 2 2 x 1 20\sqrt{x - \sqrt{2x - 1}} = 100 - 2\sqrt{2x - 1} 10 x 2 x 1 = 50 2 x 1 10\sqrt{x - \sqrt{2x - 1}} = 50 - \sqrt{2x - 1}

Square again,

100 x 100 2 x 1 = 2500 100 2 x 1 + 2 x 1 100x - 100\sqrt{2x - 1} = 2500 - 100\sqrt{2x -1} + 2x - 1

100 x = 2500 + 2 x 1 100x = 2500 + 2x - 1

98 x = 2499 98x = 2499

x = 2499 98 = 51 2 x = \frac{2499}{98} = \frac{51}{2}

Thus a + b = 51 + 2 = 53 a + b = 51 + 2 = 53

Roihan Munajih
Aug 6, 2013

x + 2 x 1 + x 2 x 1 = 10 \sqrt{x+\sqrt{2x-1}}+\sqrt{x-\sqrt{2x-1}}=10 \rightarrow Square both sides, we get x + 2 x 1 + x 2 x 1 + 2 ( x + 2 x 1 ) ( x 2 x 1 ) = 100 x+\sqrt{2x-1}+x-\sqrt{2x-1}+2\sqrt{(x+\sqrt{2x-1})(x-\sqrt{2x-1})}=100 \Leftrightarrow 2 x + 2 x 2 2 x + 1 = 100 2x+2\sqrt{x^2-2x+1}=100 \Leftrightarrow 2 x + 2 ( x 1 2 ) = 100 2x+2\sqrt{(x-1^2)}=100 \Leftrightarrow 2 x + 2 ( x 1 ) = 100 2x+2(x-1)=100 \Leftrightarrow 4 x = 102 4x=102 \Leftrightarrow x = 102 4 = 51 2 x=\frac{102}{4}=\frac{51}{2} So, a + b = 51 + 2 = 53 a+b=51+2=53

Yusrin C'orbyt
Aug 6, 2013

√(x+√(2x-1)) +√(x-√(2x-1)) = 10

〖(√(x+√(2x-1)) +√(x-√(2x-1)) )〗^2= 〖10〗^2

x + √(2x-1) + 2.√((x+√(2x-1)).(x-√(2x-1)) ) + x - √(2x-1) = 100

2x + 2(√(x^2-2x+1)) = 100

2x + 2(x-1) = 100

x + x-1 = 50

2x = 51

x = 51/2

a = 51, b = 2

a+b = 51 + 2 = 53

Jian Feng Gao
Aug 6, 2013

Squaring both sides of the equation gives

2x + 2root(x^2-2x+1)=100

2x+2(x-1)=100

4x=102

x=51/2

Therefore 51+2 =53

wrong

aditya rungta - 7 years, 10 months ago
Fatin Farhan
Aug 5, 2013

Squaring both sides we get x + 2 x 1 + x 2 x 1 + 2 ( x + 2 x 1 ) ( x 2 x 1 ) = 1 0 2 x+\sqrt{2x-1}+x-\sqrt{2x-1}+2\sqrt{(x+\sqrt{2x-1})(x-\sqrt{2x-1})}=10^2 2 x + 2 x 2 2 x + 1 = 100 2x+2\sqrt{x^2-2x+1}=100 \Rightarrow 2 x + 2 ( x 1 ) = 100 2x+2(x-1)=100 \Rightarrow 4 x = 102 4x=102 \Rightarrow x = 51 2 x=\frac {51}{2} \Rightarrow a + b = 53 a+b=53

Vikash Kumar
Aug 5, 2013

Let (2x-1)^1/2=y =>2x-1=y^2; by putting y in the given equation... (x+y)^1/2 + (x-y)^1/2 =10 taking both side square, 2x+2*(x^2-y^2)^1/2=100; by putting value of y^2=2x-1 in the equation 2x+2(x-1)=100; =>x=51/2=a/b; a+b=53 sol.

Joseph Moore
Aug 5, 2013

Pretty simple algebra, anybody who knows how to square and knows the factors of two perfect squares can do this easily

  1. Square both sides: 100 = x + / s q r t 2 x 1 + x / s q r t 2 x 1 + 2 / s q r t x + / s q r t 2 x 1 / s q r t x / s q r t 2 x 1 100 = x + /sqrt{2x-1} + x - /sqrt{2x-1} + 2/sqrt{x + /sqrt{2x-1}}/sqrt{x - /sqrt{2x-1}}
  2. Simplify: 100 = 2 x + 2 ( x + 2 x 1 ) ( x 2 x 1 ) 100 = 2x +2\sqrt{(x+\sqrt{2x-1})(x-\sqrt{2x-1})}
  3. Divide each side by two, and multiply inside the radical : 50 = x + x 2 2 x + 1 50 = x + \sqrt{x^{2}-2x+1}
  4. Factor trinomial: 50 = x + ( x 1 ) 2 50 = x + \sqrt{(x-1)^{2}}
  5. Simplify: 50 = x + x 1 50 = x + x - 1
  6. Simplify: 51 = 2 x 51 = 2x
  7. Simplify: x = 51 / 2 = a / b x = 51/2 = a/b
  8. Solve: a + b = 51 + 2 = 53 a+b = 51+2 = 53

Moderator note:

It may be pretty simple algebra, but you must know your facts well!

For example, ( 0 1 ) 2 0 1 \sqrt{ (0-1)^2 } \neq 0 - 1 .

Hs N
Aug 5, 2013

If we square the entire expression, we get 2 x + 2 x 2 2 x + 1 = 2 x + 2 ( x 1 ) 2 = 100 2x+2\sqrt{x^2-2x+1}=2x+2\sqrt{(x-1)^2}=100 , from which the answer immediately follows.

Nam Nguyen Hoang
Aug 4, 2013

Let $ a=\sqrt{x+\sqrt{2x-1}} , b=\sqrt{x-\sqrt{2x-1}} $ We have: $m+n=10$ $mn=|x-1|$ $m^2 + n^2 =2x$ So, $2x+2|x-1|= (m+n)^2 = 10^2 =100$ So, $x=\frac{51}{2} So, a+b=53

Albert Ho
Aug 4, 2013

Square both sides to get:

2 x + 2 x 2 2 x + 1 = 100 2x + 2\sqrt{x^{2}-2x+1} = 100

2 x + 2 ( x 1 ) = 100 2x + 2(x-1) = 100

4 x = 102 4x = 102

Thus, x = 51 / 2 x = 51/2 , so our answer is 53

Ahmed Lo'ay
Aug 4, 2013

sqrt( x + sqrt(2x - 1) ) + sqrt( x - sqrt(2x - 1) ) = 10

sqrt( x + sqrt(2x - 1) ) = 10 - sqrt( x - sqrt(2x - 1) ) "by squaring both sides"

x + sqrt(2x -1) = 100 - 20*sqrt( x - sqrt(2x - 1) ) + x - sqrt(2x -1)

2 sqrt(2x -1) = 100 - 20 sqrt( x - sqrt(2x - 1) )

sqrt(2x -1) = 50 - 10*sqrt( x - sqrt(2x - 1) )

> 10*sqrt( x - sqrt(2x - 1) ) = 50 - sqrt(2x -1) "by squaring both sides"

100 ( x - sqrt(2x - 1) ) = 2500 - 100 sqrt(2x - 1) + 2x - 1

100x - 100 sqrt(2x - 1) = 2500 - 100 sqrt(2x - 1) + 2x - 1

> 98x = 2499

>>>> x = 2499/98 = 51/2 = a/b

then a+b = 53

Jiaqi Wang
Aug 4, 2013

( x + 2 x 1 + x + 2 x 1 ) 2 = 4 x 2 = 100 (\sqrt{x + \sqrt{2x-1}} +\sqrt{x + \sqrt{2x-1}}) ^ 2 = 4x - 2 =100

x = 51 / 2 x = 51/2

Answer is 51 + 2 = 53

Ruslan Abdulgani
Aug 4, 2013

Square both sides, and simplify,then we get the following expression: 2(x^2-(2x-1))^0.5 + 2x = 100, then (x-1) + x = 50, So x=51/2 = a/b. So a+b = 53

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...