解：cos(a+b)cos(a-b)=1/3(cosacosb-sinasinb)(cosacosb+sinasinb)=1/3cos²acos²b-sin²asin²b=1/3cos²a(1-sin²b)-(1-cos²a)sin²b=1/3cos²a-cos²a sin²b-sin²b+cos²asin²b=1/3cos²a-sin²b=1/3
