sin(A + B)
= sinAcosB + cosAsinB
sin(A - B)
= sinAcosB - cosAsinB
则:
sin(A + B)× sin(A - B)
= (sinAcosB + cosAsinB)x (sinAcosB - cosAsinB)
= sin²Acos²B - cos²Asin²B
= sin²A(1 - sin²B) - sin²B(1 - sin²A)
= sin²A - sin²Asin²B - sin²B + sin²Asin²B
= sin²A - sin²B
追问很完美,懂了