# If from an external point B of a circle with center O, two tangents BC and BD are drawn such that ∠DBC = 120°, prove that BC + BD = BO i.e., BO = 2 BC.

Given : BC and BD are two tangents drawn from a common external point to a circle with center O such that DBC = 120°

To Prove : BC + BD = BO i.e. BO = 2BC

Proof :

In OBC and ODB

OC = OC [ Radii of same circle ]

OB = OB [Common]

BC = BD [Tangents drawn from an external point to a circle are equal ] [1]

OBC OBD [ By Side Side Criterion ]

CBO = DBO [Corresponding parts of congruent triangles are equal ]

Also,

DBC = 120°

CBO + DBO = 120°

CBO + CBO = 120°

CBO = DBO = 60°

OC BC [Tangent at a point on the circle is perpendicular to the radius through point of contact ]

Therefore,

OBC is a right-angled triangle at C

So we have,

BO = 2BC

From [1] we have

BC = BD

BC + BC = BC + BD

2BC = BC + BD

BO = BC + BD

Hence Proved .

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