Rationalizing may help

Algebra Level 2

Solve the following equation ( x + 1 ) 1 2 ( x 1 ) 1 2 = 1 (x+1)^{\frac{1}{2}} - (x-1)^{\frac{1}{2}} = 1


The answer is 1.25.

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5 solutions

Daniel Liu
Jun 11, 2014

A different solution is as follows:

Let a 2 = x + 1 a^2=x+1 and b 2 = x 1 b^2=x-1 .

We see that a 2 b 2 = 2 a^2-b^2=2 . Also, from the equation, a b = 1 a-b=1 .

Dividing the second equation from the first we get a + b = 2 a+b=2 .

Adding this to the second equation and simplifying, we get a = 3 2 a=\dfrac{3}{2} so b = 1 2 b=\dfrac{1}{2} .

Thus x 1 = 1 4 x-1=\dfrac{1}{4} so x = 5 4 = 1.25 x=\dfrac{5}{4}=\boxed{1.25} .

Very neat. But there is no need to solve for b. You can find x from a = 3/2.

D.W. Read - 6 years, 12 months ago

very intelligent solution.But we can calculate x from 'a' value

Nathani Venkateswara Rao - 6 years, 11 months ago

I saw a similar problem on aops recently. I had the same solution.

Nathan Ramesh - 7 years ago

I didn't understood

Anuj Shikarkhane - 6 years, 12 months ago

lol...i keep typing 12.5......

Zack Yeung - 5 years, 10 months ago

not level four sum!!

Sanyog Ghosh - 6 years, 11 months ago
Jubayer Nirjhor
Jun 11, 2014

Squaring and rearranging gives 2 x 1 = 2 x 2 1 2x-1=2 \sqrt{x^2-1} . Squaring again, canceling and rearranging gives 4 x 5 = 0 4x-5=0 , that is, x = 5 / 4 = 1.25 x=5/4=1.25 . It's easy to check that this value indeed works.

How does squaring and rearranging give 2x-1? Shouldn't it be 2x^2-2? Am I wrong???

Jayakumar Krishnan - 7 years ago

Did in the same way!!

Anik Mandal - 7 years ago

Impressive

Chandrachur Banerjee - 7 years ago
Tan Li Xuan
Jun 16, 2014

First,we change the equation to x + 1 x 1 = 1 \sqrt{x+1} - \sqrt{x-1} = 1 Then,we rearrange the terms to get x + 1 1 = x 1 \sqrt{x+1} - 1 = \sqrt{x-1} Squaring both sides,we get x + 1 2 x + 1 + 1 = x 1 x + 1 - 2\sqrt{x+1} + 1 = x -1 Subtracting the x x from both sides,we get 2 2 x + 1 = 1 2 - 2\sqrt{x+1} = -1 Then,we get 2 x + 1 = 3 2\sqrt{x+1} = 3 .Dividing by 2,we get x + 1 = 3 2 \sqrt{x+1} = \frac{3}{2} .Squaring both sides,we get ( x + 1 ) = 9 4 (x+1) = \frac{9}{4} So, x = 5 4 = 1.25 x = \frac{5}{4} = 1.25

Thanks :) ....

Jayakumar Krishnan - 6 years, 11 months ago

A common solution is as follows:

{ (x+1) }^{ 1/2 }=1+{ (x-1) }^{ 1/2 }

squaring both sides to the above equation,

(x+1)=1+x-1+2{(x-1)}^{1/2}

1=2{(x-1)}^{1/2}

again squaring the both sides to the above equation,

1=4(x-1)

x=5/4.

Hey yo,

(x+1)^0.5 = 1 + ( x - 1)^0.5

Squaring both sides,

1 = 2(x-1)^(0.5)

(x-1)^(0.5) = 1/2,

Squaring both sides once again,

x = 1/4 + 1 = 1.25.....(try to insert the value to check the answers)

(1.25 + 1)^(0.5) - ( 1.25 -1 )^(0.5) = 1 (yeah x = 1.25)....

thanks.....

yea same. But the soln would be better with LaTeX formatting

Jackal Jim - 6 years, 11 months ago

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