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Question
In ∆ABC, if a = 13, b = 14, c = 15, then find the value of cos B
Solution
By cosine rule, we have
cos B = `("c"^2 +"a"^2 - "b"^2)/(2"ca")`
∴ cos B = `(15^2 + 13^2 - 14^2)/(2 xx 15 xx 13)`
= `(225 + 169 - 196)/390`
= `198/390`
∴ cos B = `33/65`
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