I have found in Mathoverflow [Kahler cone of $\mathbb{C}P^n\sharp\mathbb{C}P^n$ ] [1] that :

Let $X=\mathbb{C}P^n\sharp\mathbb{C}P^n$ be the blowing up at a point.  The K$\ddot{a}$hler cone of $X$ is 
$$\mathcal{P}\backsimeq \{ a,b\in \mathbb{R}^2| a>0,b>0,a>b\}$$
There the author presents the proof of the above isomorphism, however I cannot understand. Can anyone offer more explicit proof or some references of this problem? Thank you very much! 


  [1]: https://mathoverflow.net/questions/96118/what-does-the-k%C3%A4hler-cone-of-the-one-point-blow-up-of-mathbbcpn-look-like