Пошаговое объяснение:
1) 2С = 2π*R
R = 2C/2π = C/π
2) 2C = 7R*9π
R = 2C/63π
3) πR = 2C*2π
R = 2C*2π/π = 4C
4) π/R = 2C*2π
π = 4Cπ*R
R = π/4Cπ = 1/4C (С - тоже в знаменателе)
5) R/2C = 2π/7
R = 2π*2C/7 = 4πC/7
