Question

if a=2, b=3, c=4, prove that cos A=7/8​

Answer 1

Explanation:

The first form of the Law of Cosines :

a2=b2+c2−2(b)(c)cos(A)

Solve for cos(A):

cos(A)=a2−b2−c2−2bc

Substitute, 4 for a, 6 for b, and 8 for c:

cos(A)=42−62−82−2(6)(8)

cos(A)=−84−96

cos(A)=78