c++继承提问

Square的实现不对，请问是哪一步错了
//定义一个基类shape，在此基础上派生出Rectangle和Circle，二者都有GetArea()函数计算对象的面积。
//再使用Rectangle类创建一个派生类Square。
#include<iostream>
using namespace std;
class shape
{
protected:
float a,b;
};

class Rectangle:public shape
{
public:
Rectangle(float a,float b)
{
this->a=a;
this->b=b;
}

void GetArea()
{
cout<<"area:"<<a*b<<endl;
}
};

class Circle:public shape
{
public:
Circle(float a)
{
this->a=a;
}
void GetArea()
{
const float PI=3.14;
cout<<"area:"<<PI*a*a<<endl;
}
};

class Square:public Rectangle
{
public:
Square(float x)
{
a=x;
b=x;
}
};
int main()
{
Rectangle a(2,3);
a.GetArea();
Circle b(2);
b.GetArea();