 Hello and welcome to the session. Let us understand the following question which says, the radius of a circle is increasing at the rate of 0.7 meter per second. What is the rate of increase of its circumference? Now let us proceed on to the solution. Let r be the radius of the circle, a be the area of the circle. It is given to us that radius of the circle is increasing at the rate of 0.7 meter per second. That is, dr by dt is equal to 0.7 and we have to find the rate of increase of its circumference. The circumference of a circle is given by c is equal to 2 pi r, where c is the circumference of the circle and r is the radius and here we have to find dc by dt. Therefore, dc by dt is equal to 2 pi dr by dt, dr by dt is equal to 0.7. So, this implies dc by dt is equal to 2 pi multiplied by 0.7, which is equal to 1.4 pi. Hence, circumference of the circle is increasing at the rate of 1.4 pi centimeter per second, which is the required answer. I hope you understood this question. Bye and have a nice day.