已知sina+sinb=1,cosa+cosb=o,求cos(a+b)的值sin²a+2sinasinb+sin²b=1cos²a+2cosacosb+cos²b=0两式相加得2cosacosb+2sinasinb+2=1cosacosb+sinasinb=-0.5cos(a-b)=-0.5
