简单问题

double wendu();看看你的声明

#include<iostream>
using namespace std;
double wendu(double );
int main()
{
    cout<<"Please enter a Celsius value:";
        double abc;
        cin>>abc;
    double h;
    h=wendu(abc);
    cout<<abc<<"degrees Celisus is="
        <<h<<"degrees Fahrenheit\n";
    return 0;
}
double wendu(double temp)
{
    return 1.8*temp+20;
}

看看基础...函数哪部分

晕，楼主你的函数学的不怎么样啊，既然是void，那么没有返回值
#include<iostream>
using namespace std;
double guang(double);
int main()
{
    cout<<"Enter the number of light years:";
        double a;
    cin>>a;
    cout<<a<<"\n";
    cout<<a<<" "<<"light years=";
    double abc;
    abc=guang(a);
    cout<<abc<<"astronmical units";
    return 0;
}
double guang(double temp)   //错误的地方
{
    return temp*63240;
}