1.

In the adjacent figure, if AB = 12 cm, BC = 8 cm and AC = 10 cm, then AD =A. 5 cmB. 4 cm C. 6 cmD. 7 cm

Answer»

Answer is D. 7 cm

Given: 

AB = 12 cm 

BC = 8 cm 

AC = 10 cm

Property: If two tangents are drawn to a circle from one external point, then their tangent segments (lines joining the external point and the points of tangency on circle) are equal. 

By the above property, 

AD = AF (tangent from A) 

BD = BE (tangent from B) 

CF = CE (tangent from C) 

Clearly,

AB = AD + DB = 12 cm

BC = BE + EC = 8 cm 

AC = AF + FC = 10 cm 

Now, 

AB – BC = 12 cm – 8 cm 

⇒ (AD + DB) – (BE + EC) = 12 cm – 8 cm 

⇒ AD + DB – BE – EC = 12 cm – 8 cm 

⇒ AD + BE – BE – CF = 12 cm – 8 cm [∵ DB = BE and CF = CE] 

⇒ AD – CF = 12 cm – 8 cm 

⇒ AD – (10 cm – AF) = 12 cm – 8 cm [∵AF + FC = 10 cm 

⇒ FC = 10 cm – AF] 

⇒ AD – (10 cm – AF) = 4 cm 

⇒ AD – 10 cm + AF = 4 cm 

⇒ AD + AD = 4 cm + 10 cm [∵ AD = AF] 

⇒ 2AD = 14 cm

⇒ AD = \(\frac{14cm}{2}\)

⇒ AD = 7 cm 

Hence, AD = 7 cm



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