S=πR²
1)R=2 , S=πR²=3.14×4=12.56
2)R=5, S=πR²=3.14×25=78.5
3)S=9, R= √S÷π=√9÷3.14≈√2.9≈1.7
4)R=2÷7, S=πR²=3.14×4÷49≈0.7
5)S=49π, R=√S÷π=√49π÷π=√49=7
6)R=54.3, S=πR²=3.14×2948.49≈9258
7)S=√3, R=√S÷π=√√3÷3.14≈√0.6≈0.8
8)R=6.25, S=πR²=3.14×39.0625=≈122.7
