On rational points of conics Let $K$ be a field of characteristic $0$. Let $C/K$ be a be a quasi-projective conic defined over $K$. Let $L/K$ be a finite dimensional field extension of odd dimension. Assume that
$C(L)$ is not empty. 
Q: Then is true that $C(K)$ is not empty? If so, then how does one prove it?
 A: You can find an elementary proof in 
Un théorème arithmétique sur les coniques by Trygve Nagell in Ark. f. Mat. 2 (1952). This is
a quadratic analogue of what Birch called Heegner's Lemma, which deals with the solvability of certain quartic equations.
A: So if we use Riemann-Roch for smooth projective curves over $K$ the problem becomes easy. So without lost of generality we may assume that $C/K$ is smooth and projective since a conic 
admits a point over $K$ iff $C(K)$ is infinite (this is because the existence of a parametrization over $K$).
If $C(K)$ is not empty we are done. So now assume that $C(K)$ is empty. We will try to reach a contradiction. Let $Q$ be a point in $C(L)$ with minimal field of definition
$L$ and let $[L:K]=m\equiv 1\pmod{2}$. Choose a point $P_1$ in $C(\overline{K})$ that lives in a quadratic extension of $K$. Such a point exists since we have a conic and $C(K)$ is empty. Let $P:=P_1+P_1^{\sigma}\in Div_K(C)$. Thus we have
$deg(P)=2$ and $deg(Q)=m\geq 3$. We way thus write $m=2a+1$ for 
some positive integer $a$. Now let
$$
D:=[Q]-a[P]\in Div_K(C)
$$
We have $deg(D)=1$. Now let us consider the line bundle $L_D$ on $C$ where
$$
L_D=\{f\in K(C):div(f)\geq -D\}
$$
By Riemann-Roch, we have that $dim_K(L_D)=2$ and thus there exists a non-constant function
$f\in L_D$. Note that the map
$$
C(\overline{K})\rightarrow P^1(\overline{K})
$$
given by $x\mapsto [f(x),1]$ has degree $deg(div(f)_{\infty})$. So in general, it is not an embedding. Now let us work over $\overline{K}$ so that $[Q]=[Q_1]+[Q_2]\ldots+[Q_m]$
and $[P]=[P_1]+[P_2]$. Since 
$$
div(f)\geq -D,
$$
$f\in K(C)$ (so $deg(div(f))=0$ and $div(f)$ is $G_K$-invariant) we must have that over $\overline{K}$
$$
div(f)=a[P_1]+a[P_2]+[P_3]-[Q_1]-[Q_2]-\ldots -[Q_m]
$$
where $deg([P_3])=1$. This forces $P_3$ to be defined over $K$. This contradicts the fact that $C(K)$ was empty.
