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If \(\sqrt{2+\sqrt{2+\sqrt{2+\sqrt{2+...}}}} = \text{cosec}\theta\), then what is \(\sin\theta\) equal to?

This question was previously asked in
CDS 2 2025 Maths Question Paper (14-Sep-2025)
The correct answer is
\(\frac{1}{2}\)

Evaluating the Infinite Nested Radical

The problem asks us to find the value of \(\sin\theta\) given the equation \(\sqrt{2+\sqrt{2+\sqrt{2+\sqrt{2+...}}}} = \text{cosec}\theta\). First, let's evaluate the infinite nested radical expression. Let the value of the expression be \(x\). \(x = \sqrt{2+\sqrt{2+\sqrt{2+\sqrt{2+...}}}}\) Notice that the expression under the square root contains the original expression itself. We can rewrite this relationship as:

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

To solve for \(x\), we can square both sides of the equation:

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

Now, rearrange this into a standard quadratic equation form (\(ax^2 + bx + c = 0\)):

\(x^2 - x - 2 = 0\)

We can solve this quadratic equation by factoring:

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

This gives us two possible solutions for \(x\): \(x=2\) or \(x=-1\). Since \(x\) represents the value of a square root (specifically, the principal square root), it must be a non-negative value. Therefore, we must choose \(x=2\). So, the value of the infinite nested radical is 2.

Relating Radical Value to Trigonometric Function

The problem states that the value of the nested radical is equal to \(\text{cosec}\theta\). We found the value of the radical to be 2. Therefore:

\(\text{cosec}\theta = 2\)

Calculating sin(theta)

We need to find the value of \(\sin\theta\). We know the fundamental relationship between \(\sin\theta\) and \(\text{cosec}\theta\):

\(\sin\theta = \frac{1}{\text{cosec}\theta}\)

Substitute the value of \(\text{cosec}\theta\) we found:

\(\sin\theta = \frac{1}{2}\)

Thus, the value of \(\sin\theta\) is \(\frac{1}{2}\).

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