Question

In ∆ABC, if a = 13, b = 14, c = 15, then find the value of cos B

Answer 1

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`