兩角和差公式
    sin(A+B)=sinAcosB+cosAsinB
    sin(A-B)=sinAcosB-sinBcosA ?
    cos(A+B)=cosAcosB-sinAsinB
    cos(A-B)=cosAcosB+sinAsinB
    tan(A+B)=(tanA+tanB)/(1-tanAtanB)
    tan(A-B)=(tanA-tanB)/(1+tanAtanB)