证明:要证 sin(a+b)sin(a-b)=(sina+sinb)(sina-sinb)只须证 (sina*cosb+cosa*sinb)(sina*cosb-cosa*sinb)=(sina+sinb)(sina-sinb)利用平方差公式只须证 sina*sina*cosb*cosb-cosa*cosa*sinb*sinb=sina*sina-sinb*sin...