محیط دایره $=2\pi R$ $60=2\times \pi \times R\Rightarrow R=\frac{60}{6}=10cm=0/1m$ $A=\pi {{R}^{2}}=3\times 0/{{1}^{2}}=0/03{{m}^{2}}$ $\Phi =BA\operatorname{Cos}\theta =0/4\times 0/03\times \operatorname{Cos}{{0}^{{}^\circ }}=0/012Wb$