First, read the English article below.

The formula is C=2πr{\displaystyle C=2\pi r}, where C{\displaystyle C} equals the circle’s circumference, and r{\displaystyle r} equals its radius. The symbol π{\displaystyle \pi } ("pi") is a special number, roughly equal to 3.14. You can either use that estimate (3.14) in calculations, or use the π{\displaystyle \pi } symbol on a calculator. Use algebra to change the circumference formula until r (radius) is alone on one side of the equation: Whenever a math problem tells you the circumference C of a circle, you can use this equation to find the radius r. Replace C in the equation with the circumference of the circle in your problem: Enter your result in a calculator with the π{\displaystyle \pi } button and round the result. If you don't have a calculator, calculate it by hand, using 3.14 as a close estimate for π{\displaystyle \pi }. 

Now, please write a short abstract for it in Vietnamese. Abstract:
Viết công thức tính chu vi hình tròn. Tính r (bán kính). Thay giá trị chu vi vào công thức. Làm tròn thành đáp số thập phân.