Problem: A running track is the ring formed by two concentric circles. It is 101010 feet wide. The circumferences of the two circles differ by about
Answer Choices:
A. 101010 feet
B. 303030 feet
C. 606060 feet
D. 100100100 feet
E. none of these Solution:
C1=2πr,C2=2π(r+10);∴C2−C1=20π∼60C_{1}=2 \pi r, C_{2}=2 \pi(r+10) ; \quad \therefore C_{2}-C_{1}=20 \pi \sim 60C1​=2πr,C2​=2π(r+10);∴C2​−C1​=20π∼60 (feet).