1/15+1/35+1/63+1/99+1/11=1/(3×5)+1/(5×7)+1/(7×9)+1/(9×11)+1/11=1/2×(1/3-1/5)+1/2×(1/5-1/7)+1/2×(1/7-1/9)+1/2×(1/9-1/11)+1/11=1/2×(1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)+1/11=1/2×(1/3-1/11)+1/11=1/2×8/33+1/11=7/33