Question

三角转换,请问探究活动的第二行怎么到第三行的,第三行怎么到第四行的

Answer 1

cos2A+cos2B

=cos[(A+B)+(A-B)]+cos[(A+B)-(A-B)]

=[cos(A+B)cos(A-B)-sin(A+B)sin(A-B)]+[cos(A+B)cos(A-B)+sin(A+B)sin(A-B)]

=2cos(A+B)cos(A-B),

——》原式=1-(cos2A+cos2B)/2+(1-cos^2C)

=2-2cos(A+B)cos(A-B)-cos^2C.