A shark(鲨鱼)sees a monkey. It says, “Hello, my friend, would you please come to my house? My mother and the others of my family want to see you.”

    “That's very good.” the monkey says, “But I can't swim.”

    “Oh, sit on my back. I'll take you there.” On the way, the shark says, “I must tell you my mother is ill, she wants to eat your heart.”

    “But I'm sorry, I don't bring it with me. I put it in the tree. Take me back and get it.” says the monkey.

    “Let's go back.” shark says.

    When they return, the monkey climbs up the tree and leaves the shark waiting for him in the water.

已知$\frac{b}{a}=\frac{2}{3}$，则$\frac{a}{a+b}$的值是（　　）