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Question

Two tangents, PQ and QR, are drawn from an external point Q to a circle of centre O. If OQ = 41 cm, $∠PQR=60^0$ and the radius of the circle is 9 cm, find the perimeter of $ΔPRQ$


 

This question was previously asked in
RRB NTPC 2025 Graduate CBT 2 Question Paper PDF (10-Jul-2026) (Shift 1)
The correct answer is

120 cm

The question involves two tangents, PQ and QR, drawn from an external point Q to a circle with center O. We are given the following data:

  • OQ (distance from the center to the external point Q) = 41 cm
  • ∠PQR = 60°
  • Radius of the circle = 9 cm

We need to calculate the perimeter of ΔPRQ.

Let's solve this step-by-step:

  1. From the property of tangents drawn from an external point to a circle, the lengths of tangents from a single external point are equal. Thus, PQ = QR.
  2. In the triangle ΔOQP and ΔOQR, both are right triangles because the radius is perpendicular to the tangent at the point of contact. Hence, ∠OQP = ∠OQR = 90°.
  3. We have OQ = 41 cm and OP = OR = 9 cm (radius).
  4. Using the Pythagorean theorem in ΔOQP:
  5. OP^2 + PQ^2 = OQ^2
  6. Substituting the given values: 9^2 + PQ^2 = 41^2
  7. Calculating further: 81 + PQ^2 = 1681
  8. Therefore, PQ^2 = 1681 - 81 = 1600
  9. It follows that PQ = \sqrt{1600} = 40 \text{ cm}
  10. We have found that PQ = QR = 40 cm.
  11. Now we calculate the perimeter of ΔPRQ:
  12. Perimeter = PQ + QR + PR = 40 cm + 40 cm + PR
  13. Considering ΔPQR where ∠PQR = 60°, we know PR is opposite this angle.
  14. Since PQ = QR, ΔPQ is an isosceles triangle. Let PR = x:
  15. We apply the cosine rule in ΔPQR: PR^2 = PQ^2 + QR^2 - 2 \cdot PQ \cdot QR \cdot \cos(60°)
  16. Substitute the values: PR^2 = 40^2 + 40^2 - 2 \times 40 \times 40 \times \frac{1}{2}
  17. Simplifying: PR^2 = 1600 + 1600 - 1600 = 1600
  18. This gives us PR = \sqrt{1600} = 40 \text{ cm}
  19. Finally, the Perimeter of ΔPRQ: 40 + 40 + 40 = 120 \text{ cm}

Therefore, the perimeter of ΔPRQ is 120 cm.

Hence, the correct answer is 120 cm.

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