In a ` A B C` , if `a=3,b=5 and c=7`,find `cos A ,cos B and cos C`

Explanation

We know by cosine formula that

`cos A = (b^2+c^2−a^2)/(2bc) `
Putting given values of a,b,c
=`(5^2+7^2−3^2)/(2.5.7) `
=`(25+49−9)/70 `

=`65/70 =13/14`


`cos B = (a^2+c^2−b^2)/(2ac) `
Putting given values of a,b,c
=`(3^2+7^2−5^2)/(2.3.7)`
=`(9+49−25)/(42 )`

=`33/42 =11/14 `


`cos C = (a^2+b^2−c^2)/(2ab)`
Putting given values of a,b,c
=`(3^2+5^2−7^2)/(2.3.5) `
=`(9+25−49)/30`

=`−15/30=-1/2`