Combine absolute path and relative path to get a new absolute path

I'm writing a program in which one of the components must be able to take a path it is given (such as /help/index.html, or /help/) and a relative path based on that location, (such as ../otherpage/index.html, or sub/dir/of/help/, or help2.html) and produce the absolute path implied by the relative path. Consider the following directory tree.

/
index.html
content.txt
help/
    help1.html
    help2.html

The file index.html contains a link like help/help1.html. The program is passed / or /index.html, and combines it with help/help1.html to get /help/help1.html.

Similarly, the file /help/help1.html has the link ../content.txt, from which the program needs to return /content.txt. Is there a reasonable way to do this?

Thank you. :)

Edit: Thank you to Stephen Weinberg! For everyone from the future, here's the code I used.

func join(source, target string) string {
    if path.IsAbs(target) {
        return target
    }
    return path.Join(path.Dir(source), target)
}