1/2＋1/2×1/2＋1/2×1/2×1/2＋1/2×1/2×1/2×1/2＋1/2×1/2×1/2×1/2×1/2
＝1/2＋1/4＋1/8＋1/16＋1/32
＝1－1/32
＝31/32