½(cos2B-cos2A)=sin(A+B)sin(A-B)

2个回答

  • 这是一个公式.可以利用和差角公式进行证明,

    (1/2)(cos2B-cos2A)

    =(1/2){cos[(A+B)-(A-B)]-cos[(A+B)+(A-B)]}

    =(1/2)[cos(A+B)cos(A-B)+sin(A+B)sin(A-B)]

    -(1/2)[cos(A+B)cos(A-B)-sin(A+B)sin(A-B)]

    =(1/2)sin(A+B)sin(A-B)+(1/2)sin(A+B)sin(A-B)

    =sin(A+B)sin(A-B)