#include <iostream. h> doublerectangle area( double a, double b) void mainO i double length, width cout<<"please input the length and width of the rectange cin>>length>>width cout<<the area of the rectangle is"<<rectangle area(length, width) cout<<endl doublerectangle area( double a, double b) doubles s-a return S,#include <iostream.h> double rectangle_area(double a,double b); void main() { double length,width; cout<<"please input the length and width of the rectange:"; cin>>length>>width; cout<<"the area of the rectangle is"<<rectangle_area(length,width); cout<<endl; } double rectangle_area(double a,double b) { double s; s=a*b; return s; }