In the circle below, chord  is extended to meet the tangent  at D. If  and  find the length of .

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RRB ALP Previous Paper 5 (Held On: 10 Aug 2018 Shift 3)
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  1. 6 cm
  2. √30 cm
  3. 4 cm
  4. 5 cm

Answer (Detailed Solution Below)

Option 3 : 4 cm
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Detailed Solution

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Concept Used:

According to the tangent secant theorem

DE= DB × DA

Calculation:

DB × DA = DE2

⇒ DB × (5 + DB) = 62

⇒ DB × (5 + DB) = 36

⇒ 5DB + DB2 = 36

⇒ DB2 + 5DB - 36 = 0

Solving the above quadratic equation,

DB = (-9) or DB = 4

As length can't be negative, DB = 4 cm

∴ The length of DB is 4 cm.

Shortcut Trick

DB × (5 + DB) = 36

By checking the options, we can solve this equation in less time

Thus, Option 03 satisfies the equation

∴ The length of DB is 4 cm.

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