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Q.

In a ∆ABC, if D is the mid point of BC and AD is perpendicular to AC, then cosB =

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a

c2+a2ca

b

2ba

c

-bc

d

b2+c2ca

answer is C.

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Detailed Solution

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cos⁡C=ba2⇒a2+b2-c22ab=2ba ∴a2+b2-c2=4b2 ⇒a2=c2+3b2 cosB=c2+a2-b22ca=c2+c2+3b2-b22ca=b2+c2ac
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