diff --git a/【实践课外】4.循环结构1/7-1 最大公约数和最小公倍数.c b/【实践课外】4.循环结构1/7-1 最大公约数和最小公倍数.c
new file mode 100644
index 0000000..d53067a
--- /dev/null
+++ b/【实践课外】4.循环结构1/7-1 最大公约数和最小公倍数.c	
@@ -0,0 +1,20 @@
+#include <stdio.h>
+
+int gcd(int x, int y) {
+    if (x == 0) return y;
+    return gcd(y % x, x);
+}
+
+int lcm(int x, int y) {
+    return x / gcd(x, y) * y;
+}
+
+int main() {
+    int m, n;
+
+    scanf("%d%d", &m, &n);
+
+    printf("%d %d\n", gcd(m, n), lcm(m, n));
+
+    return 0;
+}
diff --git a/【实践课外】4.循环结构1/7-1 最大公约数和最小公倍数.jpg b/【实践课外】4.循环结构1/7-1 最大公约数和最小公倍数.jpg
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diff --git a/【实践课外】4.循环结构1/7-2 判断一个整数是否为素数.c b/【实践课外】4.循环结构1/7-2 判断一个整数是否为素数.c
new file mode 100644
index 0000000..249c367
--- /dev/null
+++ b/【实践课外】4.循环结构1/7-2 判断一个整数是否为素数.c	
@@ -0,0 +1,28 @@
+#include <stdio.h>
+
+int main() {
+    int x;
+
+    scanf("%d", &x);
+
+    if (x <= 1) {
+        printf("No\n");
+    } else {
+        int is_prime = 1;
+
+        for (int i = 2; i <= x / i; i++) {
+            if (x % i == 0) {
+                is_prime = 0;
+                break;
+            }
+        }
+
+        if (is_prime == 1) {
+            printf("Yes\n");
+        } else {
+            printf("No\n");
+        }
+    }
+
+    return 0;
+}
diff --git a/【实践课外】4.循环结构1/7-2 判断一个整数是否为素数.jpg b/【实践课外】4.循环结构1/7-2 判断一个整数是否为素数.jpg
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diff --git a/【实践课外】4.循环结构1/7-3 循环-最小值.c b/【实践课外】4.循环结构1/7-3 循环-最小值.c
new file mode 100644
index 0000000..fd73acb
--- /dev/null
+++ b/【实践课外】4.循环结构1/7-3 循环-最小值.c	
@@ -0,0 +1,21 @@
+#include <stdio.h>
+
+int main() {
+    int n, min = 2147483647;
+
+    scanf("%d", &n);
+
+    for (int i = 0; i < n; i++) {
+        int x;
+
+        scanf("%d", &x);
+
+        if (x < min) {
+            min = x;
+        }
+    }
+
+    printf("%d\n", min);
+
+    return 0;
+}
diff --git a/【实践课外】4.循环结构1/7-3 循环-最小值.jpg b/【实践课外】4.循环结构1/7-3 循环-最小值.jpg
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diff --git a/【实践课外】4.循环结构1/7-4 最佳情侣身高差.c b/【实践课外】4.循环结构1/7-4 最佳情侣身高差.c
new file mode 100644
index 0000000..7702863
--- /dev/null
+++ b/【实践课外】4.循环结构1/7-4 最佳情侣身高差.c	
@@ -0,0 +1,22 @@
+#include <stdio.h>
+
+int main() {
+    int n;
+
+    scanf("%d\n", &n);
+
+    for (int i = 1; i <= n; i++) {
+        char c;
+        double h;
+
+        scanf("%c %lf\n", &c, &h);
+
+        if (c == 'M') {
+            printf("%.2lf\n", h / 1.09);
+        } else {  // c == 'F'
+            printf("%.2lf\n", h * 1.09);
+        }
+    }
+
+    return 0;
+}
diff --git a/【实践课外】4.循环结构1/7-4 最佳情侣身高差.jpg b/【实践课外】4.循环结构1/7-4 最佳情侣身高差.jpg
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diff --git a/【实践课外】4.循环结构1/7-5 循环-求和.c b/【实践课外】4.循环结构1/7-5 循环-求和.c
new file mode 100644
index 0000000..39fe503
--- /dev/null
+++ b/【实践课外】4.循环结构1/7-5 循环-求和.c	
@@ -0,0 +1,19 @@
+#include <stdio.h>
+
+int main() {
+    int m, n, s1 = 0, s2 = 0;
+
+    scanf("%d%d", &m, &n);
+
+    for (int i = m; i <= n; i++) {
+        if (i % 2 == 1) {
+            s1 += i * i;
+        } else {  // i % 2 == 0
+            s2 += i * i * i;
+        }
+    }
+
+    printf("%d %d\n", s1, s2);
+
+    return 0;
+}
diff --git a/【实践课外】4.循环结构1/7-5 循环-求和.jpg b/【实践课外】4.循环结构1/7-5 循环-求和.jpg
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diff --git a/【实践课外】4.循环结构1/7-6 循环-各位之和.c b/【实践课外】4.循环结构1/7-6 循环-各位之和.c
new file mode 100644
index 0000000..a3d2403
--- /dev/null
+++ b/【实践课外】4.循环结构1/7-6 循环-各位之和.c	
@@ -0,0 +1,17 @@
+#include <stdio.h>
+
+int main() {
+    long long n;
+    int ans = 0;
+
+    scanf("%lld", &n);
+
+    while (n != 0) {
+        ans += n % 10;
+        n /= 10;
+    }
+
+    printf("%d\n", ans);
+
+    return 0;
+}
diff --git a/【实践课外】4.循环结构1/7-6 循环-各位之和.jpg b/【实践课外】4.循环结构1/7-6 循环-各位之和.jpg
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diff --git a/【实践课外】4.循环结构1/7-7 输出闰年.c b/【实践课外】4.循环结构1/7-7 输出闰年.c
new file mode 100644
index 0000000..5d90c04
--- /dev/null
+++ b/【实践课外】4.循环结构1/7-7 输出闰年.c	
@@ -0,0 +1,27 @@
+#include <stdio.h>
+
+int main() {
+    int year;
+
+    scanf("%d", &year);
+
+    if (2000 < year && year <= 2100) {
+        int count = 0;
+
+        for (int i = 2001; i <= year; i++) {
+            if ((i % 4 == 0 && i % 100 != 0) || i % 400 == 0) {
+                printf("%d\n", i);
+
+                count++;
+            }
+        }
+
+        if (count == 0) {
+            printf("None\n");
+        }
+    } else {
+        printf("Invalid year!\n");
+    }
+
+    return 0;
+}
diff --git a/【实践课外】4.循环结构1/7-7 输出闰年.jpg b/【实践课外】4.循环结构1/7-7 输出闰年.jpg
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diff --git a/【实践课外】4.循环结构1/7-8 循环-奇数分之一序列前N项和.c b/【实践课外】4.循环结构1/7-8 循环-奇数分之一序列前N项和.c
new file mode 100644
index 0000000..54acf9c
--- /dev/null
+++ b/【实践课外】4.循环结构1/7-8 循环-奇数分之一序列前N项和.c	
@@ -0,0 +1,16 @@
+#include <stdio.h>
+
+int main() {
+    int n;
+    double ans = 0;
+
+    scanf("%d", &n);
+
+    for (int i = 0; i < n; i++) {
+        ans += 1.0 / (i * 2 + 1);
+    }
+
+    printf("%.2lf\n", ans);
+
+    return 0;
+}
diff --git a/【实践课外】4.循环结构1/7-8 循环-奇数分之一序列前N项和.jpg b/【实践课外】4.循环结构1/7-8 循环-奇数分之一序列前N项和.jpg
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