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If \(x=2+2^{1/2}+2^{3/2}\), then what is \(x^2-4x-10\) equal to?

This question was previously asked in
CDS 2 2025 Maths Question Paper (14-Sep-2025)
The correct answer is
4

Evaluating Algebraic Expressions with Radicals

We are asked to find the value of the expression \(x^2-4x-10\) given the value of \(x\). The value of \(x\) is given as:

\(x = 2 + 2^{1/2} + 2^{3/2}\)

First, let's simplify the expression for \(x\). Recall that \(a^{m/n} = \sqrt[n]{a^m}\). So, \(2^{1/2} = \sqrt{2}\) and \(2^{3/2} = \sqrt{2^3} = \sqrt{8} = \sqrt{4 \times 2} = 2\sqrt{2}\).

Substitute these simplified terms back into the expression for \(x\):

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

Now, combine the terms with \(\sqrt{2}\):

\(x = 2 + (1+2)\sqrt{2}\) \(x = 2 + 3\sqrt{2}\)

Simplifying the Expression \(x^2-4x-10\)

Our goal is to find the value of \(x^2-4x-10\). We can manipulate the simplified expression for \(x\) to find the value of \(x^2-4x\) more easily.

From \(x = 2 + 3\sqrt{2}\), subtract 2 from both sides:

\(x - 2 = 3\sqrt{2}\)

Now, square both sides of the equation to eliminate the square root:

\((x - 2)^2 = (3\sqrt{2})^2\)

Expand both sides:

\(x^2 - 2(x)(2) + 2^2 = 3^2 \times (\sqrt{2})^2\) \(x^2 - 4x + 4 = 9 \times 2\) \(x^2 - 4x + 4 = 18\)

To find the value of \(x^2-4x\), subtract 4 from both sides:

\(x^2 - 4x = 18 - 4\) \(x^2 - 4x = 14\)

Calculating the Final Value

Now we can substitute the value of \(x^2 - 4x\) into the expression we need to evaluate, which is \(x^2-4x-10\).

\(x^2 - 4x - 10 = (x^2 - 4x) - 10\)

Substitute \(x^2 - 4x = 14\):

\(= 14 - 10\) \(= 4\)

Therefore, the value of the expression \(x^2-4x-10\) is 4.

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