304 | function gauss_jordan(A, n) result(x) | 1 Error: Symbol ‘n’ at (1) has no IMPLICIT type

这个错误的原因是Fortran90中`n`变量没有声明类型，可以通过在函数声明中添加`integer, intent(in) :: n`语句来解决。修改后的代码如下：

function gauss_jordan(A, n) result(x)
implicit none
real, dimension(n,n+1), intent(inout) :: A
real, dimension(n) :: x
integer, intent(in) :: n
integer :: i, j, k, l
real :: temp

do i = 1, n
    l = i
    do j = i+1, n
        if (abs(A(j,i)) > abs(A(l,i))) then
            l = j
        end if
    end do
    if (l /= i) then
        do k = 1, n+1
            temp = A(i,k)
            A(i,k) = A(l,k)
            A(l,k) = temp
        end do
    end if
    do j = i+1, n
        A(j,i) = A(j,i)/A(i,i)
        do k = i+1, n+1
            A(j,k) = A(j,k) - A(j,i)*A(i,k)
        end do
    end do
end do
do i = n, 1, -1
    do j = i+1, n
        A(i,n+1) = A(i,n+1) - A(i,j)*x(j)
    end do
    x(i) = A(i,n+1)/A(i,i)
end do

end function

在函数声明中，我们添加了`integer, intent(in) :: n`，以声明变量`n`的类型并指定它是一个输入参数。这将消除该错误并使函数能够正常运行。