Question

since ( C=pi-A-B ), we want to maximize the
function:
( f(A, B)=cos (A)+cos (B)+cos (pi-A-B) )
( f(A, B)=cos (A)+cos (B)-cos (A+B) )
within the region R:
( A+B

# 9. In any triangle ABC, prove that cos A + cosB + cos C NIC

Solution