已知向量a=(cosa,sina),向量b=(cosb,sinb),Ia-bI的模=（2根号5）/5,（1）cos(a-b)的值为?

a-b=(cosa-cosb,sina-sinb)
|a-b|^2=(cosa-cosb)^2+(sina-sinb)^2=2-2(cosacosb+sinasinb)=4/5
cosacosb+sinasinb=3/5
cos(a-b)=cosacosb+sinasinb=3/5
-π/2