设是由圆周所围成的闭区域，则．https://img-ph-mirror.nosdn.127.net/7fwte4zRVRFsQA6AJGLNzQ==/6599274287471067843.pnghttps://img-ph-mirror.nosdn.127.net/W-tqtF-xw0asXSap_FgPUw==/6608715793817617356.pnghttps://img-ph-mirror.nosdn.127.net/5ODMKh0LECSPRXTDbA6ZPQ==/6608626733375768416.png