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In triangle ABC, angle B is a right angle. If AC is 6 cm, and D is the mid-point of side AC, the length of BD is:

1. $4$ cm
2. $6$ cm
3. $3$ cm
4. $3.5$ cm

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(AB)2 + (BC)2 = 2((AD)2 + (BD)2)

AD = 3 since D is the midpoint of AC

(AC)2 = 2(32 + (BD)2)   ....{ (AB)2 + (BC)2 = (AC)2 using Pythagoras theorem }

$\therefore$ 62 = 2(9 + (BD)2)

On solving, we get BD = 3.

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