=1-1/3+1-1/15+1-1/35+1-1/63+1-1/99
=5-(1/1×3+1/3×5+1/5×7+1/7×9+1/9×11)
=5-1/2×(1-1/3+1/3-1/5+1/5-1/7+1/9-1/9-1/11)
=5-1/2×(1-1/11)
=50/11