Question

In `triangle KLM`, if k=4, l=5, m=8, how do you find the exact value of cos M?

Answer 1

`cosM=-23/40`

Explanation:

we need tot use the cosine rule

`a^2=b^2+c^c-2bccosA`

we are given

`k=4,l=5,m=8`

we have to find `cosM`

so the cosine rule for this problem becomes

`m^2=k^2+l^2-2klcosM`

`:.8^2=4^2+5^2-2xx4xx5cosM`

`64=41-40cosM`

rearranging for `cosM`

`(64-41)/(-40)=cosM`

`cosM=-23/40`