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Arghi
Asked: If x - 10 = √x + √10, what is the value of x?

(√x + √10)(√x - √10) = √x + √10
(√x + √10) (√x - √10 - 1) = 0
√x = √10 + 1
x = (√10 + 1)^2 = 10 + 1 + 2√10 = 11 + 2√10

IMO E
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Is there any other way to do this besides the algebra outlined above? There was no way I was recognizing that x - 10 should have been changed a^2−b^2.
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Jake7Wimmer
If \(x - 10 = \sqrt{x} + \sqrt{10}\), what is the value of x?

(A) \(1 + √10\)

(B) \(10 + √10\)

(C) \(11 + √10\)

(D) \(10 + 2√10\)

(E) \(11 + 2√10\)


Is there any other way to do this besides the algebra outlined above? There was no way I was recognizing that x - 10 should have been changed a^2−b^2.
That would be the best way in my mind. Alternatively, you could also substitute answers. Substituting E gives:

\((11 + 2√10) - 10 = \sqrt{x} + \sqrt{10}\)

\(1 + √10 = \sqrt{x}\)
Squaring gives:

\(1 + 2√10 + 10 = x\)

\(11 + 2√10 = x\)
So, we got the value of x matching E.
 ­
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ShortcutoSpeed
If \(x - 10 = \sqrt{x} + \sqrt{10}\), what is the value of x?

(A) \(1 + √10\)

(B) \(10 + √10\)

(C) \(11 + √10\)

(D) \(10 + 2√10\)

(E) \(11 + 2√10\)­


If nothing comes to mind, simply approximate.

\(x - 10 = \sqrt{x} + \sqrt{10}\)
\(x - \sqrt{x} = 10 + \sqrt{10}\)

sqrt(10) is slightly more than 3. So \(x - \sqrt{x} = 13\) approx

If x = 16, then \(x - \sqrt{x} = 16 - 4 = 12\)

If x = 17, then \(x - \sqrt{x} = 17 - 4.1 = 13\) approx
Hence x = 17 works.

So we are looking for an option that gives about 17.
Since sqrt(10) = 3.1 approx, only option (E) will give about 17.

Answer (E)

Video on estimations is here: https://youtu.be/4Wy7BrQrjkM
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KarishmaB How better to solve it? I squared both sides as (x-sqrt(x))^2 = (10+sqrt(10))^2 and got smth similar to E.
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MBAToronto2024
KarishmaB How better to solve it? I squared both sides as (x-sqrt(x))^2 = (10+sqrt(10))^2 and got smth similar to E.

Squaring with x and root(x) terms is not a good idea. Upon squaring, we get x^2, x and root(x) terms. To remove root(x) we need to square again which will give us x^4 term which is just nasty!
You need to do what gmatophobia did above if you are to use algebra. If that doesn't come to mind, then try to estimate the value as explained by me in the comment above.
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Why can't i delete (√x + √10) from both side the 1st line?
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Arghi
Asked: If x - 10 = √x + √10, what is the value of x?

(√x + √10)(√x - √10) = √x + √10
(√x + √10) (√x - √10 - 1) = 0
√x = √10 + 1
x = (√10 + 1)^2 = 10 + 1 + 2√10 = 11 + 2√10

IMO E
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Why can't i delete (√x + √10) from both side the 4th line?

gmatophobia
Arghi
If x - 10 = √x + √10, what is the value of x?

A)1 + √10
B)10 + √10
C)11 + √10
D)10 + 2√10
E)11 + 2√10

\(x^2 - y^2 = (x+y)(x-y)\)

\(x - 10 = √x + √10\)

\((\sqrt{x})^2 - (\sqrt{10})^2 = \sqrt{x} + \sqrt{10}\)

\((\sqrt{x} + \sqrt{10})(\sqrt{x} - \sqrt{10}) = \sqrt{x} + \sqrt{10}\)

\((\sqrt{x} + \sqrt{10})(\sqrt{x} - \sqrt{10}) -(\sqrt{x} + \sqrt{10}) = 0\)

\((\sqrt{x} + \sqrt{10})[(\sqrt{x} - \sqrt{10}) - 1] = 0\)

As \((\sqrt{x} + \sqrt{10})\) is positive, \((\sqrt{x} - \sqrt{10}) - 1 = 0\)

\((\sqrt{x} - \sqrt{10}) - 1 = 0\)

\(\sqrt{x} = \sqrt{10}+1\)

Squaring on both sides

\(x = 10 + 1 + 2\sqrt{10}\)

\(x = 11 + 2\sqrt{10}\)

Option E
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rafin202
Why can't i delete (√x + √10) from both side the 4th line?

gmatophobia
Arghi
If x - 10 = √x + √10, what is the value of x?

A)1 + √10
B)10 + √10
C)11 + √10
D)10 + 2√10
E)11 + 2√10

\(x^2 - y^2 = (x+y)(x-y)\)

\(x - 10 = √x + √10\)

\((\sqrt{x})^2 - (\sqrt{10})^2 = \sqrt{x} + \sqrt{10}\)

\((\sqrt{x} + \sqrt{10})(\sqrt{x} - \sqrt{10}) = \sqrt{x} + \sqrt{10}\)

\((\sqrt{x} + \sqrt{10})(\sqrt{x} - \sqrt{10}) -(\sqrt{x} + \sqrt{10}) = 0\)

\((\sqrt{x} + \sqrt{10})[(\sqrt{x} - \sqrt{10}) - 1] = 0\)

As \((\sqrt{x} + \sqrt{10})\) is positive, \((\sqrt{x} - \sqrt{10}) - 1 = 0\)

\((\sqrt{x} - \sqrt{10}) - 1 = 0\)

\(\sqrt{x} = \sqrt{10}+1\)

Squaring on both sides

\(x = 10 + 1 + 2\sqrt{10}\)

\(x = 11 + 2\sqrt{10}\)

Option E

Yes, since (√x + √10) cannot be 0, we can directly reduce (divide)

(√x + √10)(√x − √10) = √x + √10

by (√x + √10) to get

√x − √10 = 1.

We cannot reduce (divide) an equation by an expression if that expression could equal 0, as division by 0 is undefined, and doing so would result in losing a root. For example, if we had x^2 = x, reducing by x directly would give x = 1 but lose the root x = 0. Instead, we should proceed like this:

x^2 = x
x^2 − x = 0
x(x − 1) = 0

This gives both roots: x = 0 and x = 1.

In the given question, however, (√x + √10) cannot be 0 since it is the sum of non-negative (√x) and positive values (√10). Therefore, we can safely reduce (divide) the entire equation by (√x + √10) without losing any roots.
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man it was soo easy!! i just couldn't think of it :(
quizicalindian
It can be inferred from the prompt that this question wants to test use of: \(a^2-b^2=(a-b)(a+b)\)

\( x-10 = (\sqrt{x})^2 - (\sqrt{10})^2 = (\sqrt{x}-\sqrt{10})(\sqrt{x}+\sqrt{10})\)

Since the question says, \(x-10 = \sqrt{x}+\sqrt{10}\) ; cross them out in the above equation

\(\sqrt{x}-\sqrt{10}=1\)

or \(\sqrt{x}=1+\sqrt{10}\)

Then square this to get x : \(1+10+2\sqrt{10}\)

Answer E
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