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The sides BC, CA and AB of a triangle AB...

The sides BC, CA and AB of a triangle ABC are of lengths a, b and c respectively. If D is the mid-point of BC and AD is perpendicular to AC, then the value of cos A cos C is:

A

`(3(a^2-c^2))/(2ac)`

B

`(2(a^2-c^2))/(3bc)`

C

`((a^2-c^2))/(3ac)`

D

`(3(c^2-a^2))/(3ac)` .

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