级数 <img data-fr-image-pasted="true" width="75" src="data:image/png;base64,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" class="fr-fic fr-dii">(其中 <img data-fr-image-pasted="true" width="16" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAANCAYAAACgu+4kAAAABGdBTUEAALGPC/xhBQAAAAlwSFlzAAAOwwAADsMBx2+oZAAAAIxJREFUOE/lkdsKxEAIQwVDEcX//1w32imULaV9XNi8zIV4zDjyO8rwgkiJoDyyHNyrVXCfBt5LwbLXew2ERcZiWAO1zKwUPkDd4gHATgqwiF2dhYoCOkWU6YsE59ixKc1M4Au2nrasV+0z2E351XFgfMYY7zSDWqYzrM+d7DF+dzmGdIX177yA/KVEPlYqQHqwO0SrAAAAAElFTkSuQmCC" class="fr-fic fr-dii">为自然数)的和是( )?