Đáp án: `P=-\frac{119}{144}`
Giải:
`P=cos(a+b).cos(a-b)=(cosacosb-sinasinb)(cosacosb+sinasinb)`
`P=cos^2acos^2b-sin^2asinb^2`
`P=cos^2acos^2b-(1-cos^2a)(1-cos^2b)`
`P=(\frac{1}{3})^2.(\frac{1}{4})^2-[1-(\frac{1}{3})^2][1-(\frac{1}{4})^2]=-\frac{119}{144}`