$x^2+y^2=c^2$

每一个实数都能用有理数去逼近到任意精确的程度，这就是有理数的稠密性。The rational points are dense on the number axis.

每一个实数都能用有理数去逼近到任意精确的程度，这就是有理数的稠密性。The rational points are dense on the number axis.