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diff --git a/report-cls03.tex b/report-cls03.tex
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+\documentclass[xelatex,a4paper,11pt,ja=standard]{bxjsarticle}
+
+\usepackage{tex/preamble}
+\usepackage{tex/detailed-title}
+
+\reportauthor{柴田健琉}
+\reporttitle{前期第3回課題}
+\reportdate{2026年}{04月}{27日}
+\turnindate{2026年}{04月}{27日}
+\schoolname{岐阜工業高等専門学校}
+\department{電子制御工学科}
+\subject{情報処理2}
+\professor{遠藤 登}
+\studentid{2024D14}
+\seatingnum{15}
+
+\begin{document}
+    \detailedtitle{}
+
+    \section{はじめに}
+
+    この課題のプログラムは以下の環境での動作が確認されている：
+
+    \begin{itemize}
+        \item{OS: NixOS 25.11 Xantusia, Linux Kernel 7.0.0 x86\_64}
+        \item{CC: GCC 15.2.0}
+        \item{CFLAGS: \texttt{-g -O1 -Wall -Wpedantic}}
+    \end{itemize}
+
+    \section{課題1}
+
+    再帰関数を用いて1から$n$の自然数の和を求めるプログラム．
+
+    \lstinputlisting[language=C,title={課題1}]{./src/cls03/a1.c}
+
+    \subsection{実行結果}
+
+    \begin{figure}[tbh]
+        \centering
+        \includegraphics[width=11cm]{./assets/cls03-a1.png}
+        \caption{課題1の実行結果}
+    \end{figure}
+
+    \newpage
+
+    \section{課題2-4}
+
+    課題2：ユークリッド互除法を用いた最大公約数を求める関数の作成．
+
+    課題3：課題2の関数を用いて最小公倍数を求める関数の作成．
+
+    課題4：配列として与えられた分数を約分する関数の作成．
+
+    \lstinputlisting[language=C,title={課題2-4の統合プログラム}]{./src/cls03/a2-4.c}
+
+    \subsection{実行結果}
+
+    \begin{figure}[tbh]
+        \centering
+        \includegraphics[width=12cm]{./assets/cls03-a2-4.png}
+        \caption{実行結果}
+    \end{figure}
+\end{document}
diff --git a/src/cls03/Makefile b/src/cls03/Makefile
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index 0000000..a3671e7
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+++ b/src/cls03/Makefile
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+include ../common.mk
+
+PROJECT_NAME:=cls03
+
+TGTS:=$(patsubst %.c,%,$(wildcard *.c))
+
+all: $(TGTS)
+
+%: %.c
+	@mkdir -p $(BUILD_PATH)/$(PROJECT_NAME)
+	$(CC) $(CFLAGS) -I. $^ -o $(BUILD_PATH)/$(PROJECT_NAME)/$@
+
+clean:
+	$(RM) -drf $(BUILD_PATH)/$(PROJECT_NAME)
diff --git a/src/cls03/a1.c b/src/cls03/a1.c
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--- /dev/null
+++ b/src/cls03/a1.c
@@ -0,0 +1,19 @@
+#include <stdio.h>
+
+unsigned int sum_recc(unsigned int n) {
+    if (n == 0) return 0;
+    return n + sum_recc(n - 1);
+}
+
+int main(void) {
+    unsigned int x = 0;
+
+    printf("Input natural number: ");
+    (void)scanf("%u", &x);
+
+    unsigned int s = sum_recc(x);
+
+    printf("Result: %u\n", s);
+
+    return 0;
+}
diff --git a/src/cls03/a2-4.c b/src/cls03/a2-4.c
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index 0000000..20e0602
--- /dev/null
+++ b/src/cls03/a2-4.c
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+#include <stdio.h>
+
+void getTwoInts(int* a, int* b) {
+    printf("1st Int: ");
+    (void)scanf("%d", a);
+    printf("2nd Int: ");
+    (void)scanf("%d", b);
+}
+
+void printRatio(int r[]) {
+    printf("%d/%d", r[0], r[1]);
+}
+
+int gcm(int a, int b);
+int lcm(int a, int b);
+void reduce(int x[]);
+
+int gcm(int a, int b) {
+    int x = a, y = b;
+    if (x < y) {
+        x = b;
+        y = a;
+    }
+    int r = a % b;
+    if (r == 0) return b;
+    return gcm(b, r);
+}
+
+int lcm(int a, int b) {
+    int g = gcm(a, b);
+    return b * a / g;
+}
+
+void reduce(int x[]) {
+    int a = x[0], b = x[1];
+    int g = gcm(a, b);
+    x[0] = a / g;
+    x[1] = b / g;
+}
+
+int main(void) {
+    int a,b;
+
+    getTwoInts(&a, &b);
+    int g = gcm(a, b);
+    int l = lcm(a, b);
+
+    printf("Result:\ngcm = %d\nlcm = %d\n", g, l);
+
+    int r[2] = {a, b};
+    printf("Before: ");
+    printRatio(r); putchar('\n');
+    reduce(r);
+    printf("After: ");
+    printRatio(r); putchar('\n');
+
+    return 0;
+}