=1/2(2/1×3+2/3×5+1/5×7+2/7×9+2/9×11)
=1/2(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)
=1/2(1-1/11)
=1/2×10/11
=5/11