2cos² [(A-B)/2]cosB-sin(A-B)sinB+cos(A+C)=-3/5.
[1+cos(A-B)]cosB-sin(A-B)sinB+cos(180º-B)=-3/5
∴cos(A-B)cosB-sin(A-B)sinB+cosB-cosB=-3/5
∴cos[(A-B)+B]=-3/5
即cosA=-3/5
2cos² [(A-B)/2]cosB-sin(A-B)sinB+cos(A+C)=-3/5.
[1+cos(A-B)]cosB-sin(A-B)sinB+cos(180º-B)=-3/5
∴cos(A-B)cosB-sin(A-B)sinB+cosB-cosB=-3/5
∴cos[(A-B)+B]=-3/5
即cosA=-3/5