[tex]\bf 7.\\ \\ \it a)\ \ m(\stackrel\frown{AB})=360^o:3=120^o \Rightarrow L_{\stackrel\frown{AB}} =\dfrac{2\pi\cdot12\cdot120^o}{360^o}=8\pi\ cm\\ \\ b)\ \ m(\stackrel\frown{AB})=360^o:4=90^o\Rightarrow L_{\stackrel\frown{AB}} =\dfrac{2\pi\cdot12\cdot90^o}{360^o}=6\pi\ cm\\ \\ c)\ \ m(\stackrel\frown{AB})=360^o:6=60^o\Rightarrow L_{\stackrel\frown{AB}} =4\pi\ cm\\ \\ d)\ \ m(\stackrel\frown{AB})=360^o:8=45^o \Rightarrow L_{\stackrel\frown{AB}} =3\pi\ cm[/tex]
[tex]\it L_{\stackrel\frown{AB}} =\dfrac{2\pi R\cdot u}{360^o}[/tex]
