cos(a+b)=1/5,cos(a-b)=1/3且o ∵sin²(a+b)+cos²(a+b)=1 , sin²(a-b)+cos²(a-b)=1∴sin(a+b)= 2/5根号6 , sin(a-b)= 2/3根号2 cos2a=cos[(a+b)+(a-b)]=cos(a+b)cos(a-b)-sin(a+b)sin(a-b)=1/5×1/3-2/5根号6×2/3根号2=(1-8根号3)/15
