-1/2
解：因为{an}为等差数列a1+a5+a9=3*a1+12*d=π,a1+4*d=π/3,a2+a8=2a1+8d=2*π/3,cos(a2+a8)=cos(2*π/3)=-1/2.
a1+a5+a9=π推出3a5=π推出a5=π/3cos(a2+a8)=cos(2a5)=cos(2π/3)=-1/2
