If the sides of a triangle are 3,5 and 7, then find cos A

Explanation

In `triangleABC`, let a=3, b=5, c=7.
Since, c is the longest side, thus `angleC` is the greatest angle.
Now, `cos C= (a^(2)+b^(2)-c^(2))/(2ab) = (3^(2) + 5^(2)-7^(2))/(2 xx 3 xx 5) = -15/30 = -1/2`
`=cos 120^(@)`
`therefore angleC=120^(@)`
Hence, the triangle is obtuse an the obtuse angle is `120^(@)`.