diff --git a/3.cpp b/3.cpp
new file mode 100644
index 0000000..8ebccc5
--- /dev/null
+++ b/3.cpp
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+//声明抽象基类Shape，包含2个虚函数shapeName( )和perimeter( )，分别返回形状名和周长。由它派生出1个子类：Trapezoid（等腰梯形）
+//用一个非类内函数printPerimeter(Shape& s)，以基类引用作为函数形式参数，输出周长，图形的数据在创建对象时给定。
+#include <iostream>
+#include <string>
+using namespace std;
+class Shape
+{public:
+    virtual string shapeName( ){}
+    virtual double perimeter( ){}              
+};
+
+
+/**************补充Trapezoid类定义**************/
+class Trapezoid:public Shape
+{
+public:
+    Trapezoid(float i,float j,float k):a(i),b(j),c(k){} 
+    string shapeName()
+    {
+        return "Trapezoid";
+    }
+    double perimeter()
+    {
+        return a+b+2*c;
+    }
+private:
+    float a,b,c;
+};
+
+/**************补充Trapezoid类定义**************/   
+
+void printPerimeter(Shape& s)    //以基类引用作为函数形参
+{cout<<"perimeter of "<<s.shapeName( )<<" = "<<s.perimeter( )<<endl;}           
+
+int main( )
+{   double a=0, b=0, c=0;
+    cin >> a >> b >> c;       
+    Trapezoid trap(a,b,c);    
+    printPerimeter(trap);        
+    return 0;
+}