In ΔDEF, DE = 9 cm, EF = 12 cm and DF = 7 cm. If DO is the angle bisector of ∠EDF meets EF at O, then find the length of OF.

  1. 5.25 cm
  2. 4.35 cm
  3. 4.00 cm
  4. 4.25 cm

Answer (Detailed Solution Below)

Option 1 : 5.25 cm
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Detailed Solution

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Given:

DE = 9, EF = 12 cm, and DF = 7 cm

DO is the angle bisector of ∠EDF

Concept used:

In a triangle, If AD is the angle bisector of ∠BAC intersect the opposite side BC at D then

F1 Amar Kumar Anil 19-05.21 D26

By angle bisector theorem

AB/AC = BD/CD

Calculation:

DE = 9, EF = 12 cm, and DF = 7 cm

DO is the angle bisector of ∠EDF

F1 Amar Kumar Anil 19-05.21 D27

By angle bisector theorem,

DE/DF = EO/OF

⇒ 9/7 = (EF - OF)/OF

⇒ 9/7 = (12 - OF)/OF

⇒ 9 × OF = 84 - 7 × OF

⇒ 16 × OF = 84

⇒ OF = 84/16

⇒ OF = 5.25 cm

∴ The value of OF is 5.25 cm.

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