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+++ b/8.21/B-Light/Readme.md
@@ -1,5 +1,5 @@
 # 题目
-* 链接
+* https://vjudge.net/contest/511373#problem/B
 ### 题意
 如图，求L
 ![](look)
@@ -13,4 +13,4 @@ midr(mid+r)/2;
 ### 易错点
 * 起始位置
 ### 工具箱
-* 
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+* 三分 15. https://note.youdao.com/web/#/file/recent/note/WEBbda8b62456b542bf29edf7a52005e158/
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