diff --git a/8.21/B-Light/Readme.md b/8.21/B-Light/Readme.md
index cc02b16..2c96c92 100644
--- a/8.21/B-Light/Readme.md
+++ b/8.21/B-Light/Readme.md
@@ -1,12 +1,16 @@
 # 题目
 * 链接
 ### 题意
-
+如图，求L
+![](look)
 ### 做法
-
+三分
+mid=(l+r)/2;
+midr(mid+r)/2;
+远离峰值的收缩
 ### 关键词
-
+三分
 ### 易错点
-* 
+* 起始位置
 ### 工具箱
 * 
\ No newline at end of file
diff --git a/8.21/B-Light/look b/8.21/B-Light/look
new file mode 100644
index 0000000..c22648f
Binary files /dev/null and b/8.21/B-Light/look differ
diff --git a/8.21/B-Light/main.cpp b/8.21/B-Light/main.cpp
index 61c413f..1d58774 100644
--- a/8.21/B-Light/main.cpp
+++ b/8.21/B-Light/main.cpp
@@ -3,29 +3,33 @@
 #include<algorithm>
 #include <cmath>
 using namespace std;
-//const int N=1e8+5;
-//void initialise(int n)
-//{
-//
-//}
+const double eps= 1e-5;
+double H,h,D;
+double f(double d)
+{
+	return D-d+H-(H-h)*D/d;
+//	return D-d+h-(H-h)*(D-d)/d;
+}
+double sanfen(double l,double r)
+{
+	double mid,midr,ans;
+	while(fabs(r-l)>eps)
+	{
+		mid=(l+r)/2;
+		midr=(mid+r)/2;
+		if(f(mid)<f(midr))l=mid;
+		else r=midr;
+	}
+	return f(l);
+}
 int main()
 {
 	int t;
-	double H,h,D;
 	scanf("%d",&t);
 	while(t--)
 	{
 		scanf("%lf%lf%lf",&H,&h,&D);
-//        initialise(n);
-		double add=D/2-(H-h);
-		if(add>0)
-		{
-			printf("%.3lf\n",h+add);
-		}
-		else
-		{
-			printf("%.3lf\n",h);
-		}
+		printf("%.3lf\n",sanfen(D-D*h/H,D));
 	}
 	return 0;
 }
diff --git a/8.21/B-Light/tempCodeRunnerFile.cpp b/8.21/B-Light/tempCodeRunnerFile.cpp
new file mode 100644
index 0000000..02358d2
--- /dev/null
+++ b/8.21/B-Light/tempCodeRunnerFile.cpp
@@ -0,0 +1 @@
+D
\ No newline at end of file
diff --git a/8.21/E-typewriter/E.pdf b/8.21/E-typewriter/E.pdf
new file mode 100644
index 0000000..564d7e7
--- /dev/null
+++ b/8.21/E-typewriter/E.pdf
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:83f6580cee8072a9e6418879a4389386696de7e13230f6b99ee194017c5e1484
+size 117561