[b][size=150]Lingkaran [/size][/b][br][justify]Lingkaran merupakan kurva tertutup sederhana. Jika kita membuat sebuah segi-n beraturan dengan n tak terhingga maka akan membentuk sebuah lingkaran. Lingkaran dapat didefinisikan sebagai tempat kedudukan dari kumpulan titik-titik yang berjarak sama terhadap sebuah titik pusat. Jarak titik P ke titik pusat O disebut dengan jari-jari lingkaran. Diameter sebuah lingkaran merupakan dua kali jari-jari lingkaran. [/justify]Berikut adalah gambar bagian-bagian dari lingkaran.

AB = d = diameter[br]OA =OB = r = jari-jari[br]d = 2r[br][img width=107,height=19]data:image/png;base64,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[/img][br]BC = busur [br]∠BOC = sudut pusat [br]Panjang busur ACB = [img width=83,height=34]data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAFMAAAAiCAMAAAAUEhVWAAAAAXNSR0IArs4c6QAAAJ9QTFRFAAAAAAAAAAA6AABmADo6ADpmADqQAGa2OgAAOgA6OgBmOjpmOjqQOmaQOma2OpC2OpDbZgAAZgA6ZgBmZjoAZjo6ZjpmZjqQZmZmZpDbZrbbZrb/kDoAkDo6kDpmkGY6kLbbkNv/tmYAtmY6tmZmtpA6ttvbttv/tv//25A625Bm27Zm27aQ29u22////7Zm/9uQ/9u2/9vb//+2///bzky4UAAAAAF0Uk5TAEDm2GYAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAAZdEVYdFNvZnR3YXJlAE1pY3Jvc29mdCBPZmZpY2V/7TVxAAACEElEQVRIS9WWy1rbMBCFJVMSlUIgpdCSCIqtQuMYLAfr/Z+Nc0ZyICzoJ4cFnUWsJPavuRzNWKn/0zqzpOOb31of/sWi0rBCfhttlebznZm1oZq0SvXzpQruYDUCWJ3Fh/ypxSrYKddFCfoRcH4Ms6ZLsHCzIl1o8dOD/mDSjjnObv2ol4pMiTp6x3wecodM676mh/qfraqmCF38IjlYpDNukWUsQzQp8lTqwuqALNvlpzN5BUp3DH+Q2sisGLqni3V2iRBsNMaplAMFwYY7qVONP5vsEnnGS1s4PWuV18VCBSj+GxXfzfHHl0VWKj/hzU/OjBLaO6GwusHmC+0NsrmSal6/nIJ8Ubz1MtgZBSJnIhq0x5KKsoMtysZMcqsE6A6SWl6X9Zk0hj/39tctFoOU/nHd7h2shquDhWsRmiulT8Szl287zOCE0Z+v+u9MsXTGbEPgktNoEckTHGlDz8zCSi63UE94jNtPHiGJobVnMdfiWNJSz5OrD1acYZ05aVNDS8AnN3Te4Ax7ytpoatmNH2/BXqUD0ZlTafFFyZnkZ+3mckzWh9iZ36E9s/+BuyczNGxuqW6xxSNLe8QOmNbIcbAnFxqSFndfRkpWFV/fvJlzaCCbjV5+FJP5ExYkuI19tIfpQUxHYclJYY2yJ9uOB/0cM4icCu9dDYZbHbW0lz38kBrhbStpH1d+/zz2DPUOOI7hc7DtAAAAAElFTkSuQmCC[/img][br][br]Keliling = [math]2\pi r[/math][br]Luas = [math]\pi r^2[/math][br]Luas juring AOB = [img width=83,height=34]data:image/png;base64,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[/img][br][br][br][br][br][br]