Question 12

Mathematics Sequence And Series Hard

In a GP consisting of positive terms, each term equals the sum of the next two terms. Then the common ratio of the GP is:

(A) <span class="math-tex">\(\dfrac{1-\sqrt{5}}{2}\)</span>
(B) <span class="math-tex">\(\dfrac{\sqrt{5}}{2}\)</span>
(C) <span class="math-tex">\(\sqrt{5}\)</span>
(D) <span class="math-tex">\(\dfrac{\sqrt{5}-1}{2}\)</span>
✓ Correct Answer: Option D Verified Solution

📘 Step-by-Step Solution & Detailed Explanation:

Let the terms be \( a, ar, ar^2, ar^3, \ldots \) According to the given condition: \[ a = ar + ar^2 \] Dividing both sides by \( a \) (assuming \( a \neq 0 \)): \[ 1 = r + r^2 \Rightarrow r^2 + r - 1 = 0 \] Solving the quadratic equation: \[ r = \frac{-1 \pm \sqrt{1 + 4}}{2} = \frac{-1 \pm \sqrt{5}}{2} \] Since \( r \) is positive, we take the positive root: \[ r = \frac{\sqrt{5} -1}{2} \]

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