提取下列材料的要点，整合成一个单句，为“形式美”下定义。

①构成事物外形的物质材料具有自然属性（色、形、声）。

②这种自然属性（色、形、声）以及它们的组合规律（如整齐、比例、对称、均衡、反复、节奏、多样的统一等）呈现出一种审美特征。

③这种审美特征就是形式美。

已知a﹣b=2+$\sqrt{3}$，b﹣c=2﹣$\sqrt{3}$，则a²+b²+c²﹣ab﹣bc﹣ac的值为（　　）

根据首字母提示补全对话

(An old woman is standing at the side of the road, near the traffic l____ . Liu Mei is on her way to the cinema, but she stops. )

Woman: Oh, dear! What shall I do?

Liu Mei: What's the m____ , Granny? Can I help you?

Woman: I'm on my way to see my h____ . He's ill in hospital.

Liu Mei: Which hospital?

Woman: I don't know! The address was on a p____ of paper, but I can't find it now.

Liu Mei: Is it in your pocket? M____ you put it there.

     (The woman finds the paper.)

Woman: Could you tell me how I can get to the hospital? I don't k____  the way. Which is the way to the hospital?

Liu Mei: Go down this street until you r____ the second traffic lights. Turn right; at the end of the r____ you'll see the hospital. It'll take you about half an hour.

Woman: Half an hour? That's too l____ ! Is there a bus?

Liu Mei: Yes, the No. 72 bus will take you there. There's a stop right o____ the hospital. I'll go with you.

Woman: How kind you are! Thank you.

若坡面与水平面的夹角为α，则坡度i与坡角α之间的关系是（　　）