以字母"F"开头的洲是非洲（Africa）。非洲是世界上第二大的洲，也是人口第二多的洲，拥有丰富的自然资源和多样的文化。非洲地域辽阔，包括了许多国家和地区，如埃及、南非、尼日利亚等。非洲的自然风景壮观，拥有世界上最长的河流尼罗河和最高的山脉乞力马扎罗山，同时也是野生动物的天堂，拥有丰富的动植物资源。
A: Hey, have you tried that new sushi place down the street? I heard it's amazing.
B: No, I haven't. I'm more of a burger and fries kind of person.
A: Well, you should give it a try. It's always good to try new things.
B: I don't know, raw fish just doesn't sound appealing to me.
A: That's understandable, but sushi is more than just raw fish. They have vegetarian options and cooked rolls too.
B: Hmm, maybe I'll give it a shot. What's your favorite roll there?
A: Definitely the spicy tuna roll. It's got the perfect amount of kick to it.
B: Alright, I'll take your word for it. Let's go check it out this weekend.
A: Sounds like a plan! And if you're still craving burgers, we can hit up that new gourmet burger joint afterwards
角度制：360/n
弧度制：2π/n
圆心角是指在中心为O的圆中，过弧AB两端的半径构成的∠AOB， 称为弧AB所对的圆心角。圆心角等于同一弧所对的圆周角的二倍。
扇形的圆心角α的范围在0°与360°之间，即0°<α<360°。扇形是圆的一部分，由两个半径和和一段弧围成，在较小的区域被称为小扇形，较大的区域被称为大扇形。
一条圆弧和经过这条圆弧两端的两条半径所围成的图形叫扇形（半圆与直径的组合也是扇形）。显然，它是由圆周的一部分与它所对应的圆心角围成。《几何原本》中这样定义扇形：由顶点在圆心的角的两边和这两边所截一段圆弧围成的图形。