The radius of a circular disc is increasing at a constant rate of 0.003cm/s. Find the rate at which the area is increasing when the radius is 20cm.

  • Google+ icon
  • LinkedIn icon

The rate at which the area is increasing, dA/dt, can be written with terms we know or can find out easily: dA/dt=dA/dr x dr/dt.

Area of a disc, A = (pi)r^2


Rate of change of radius, dr/dt=0.003cm/s

Therefore, dA/dt=2(pi)r x 0.003

= 2(pi) x 20 x 0.003


= 0.377cm^2/s

Henry H. GCSE Maths tutor, A Level Maths tutor, GCSE Further Mathemat...

About the author

is an online A Level Maths tutor with MyTutor studying at Oxford, The Queen's College University

Still stuck? Get one-to-one help from a personally interviewed subject specialist.

95% of our customers rate us

Browse tutors

We use cookies to improve your site experience. By continuing to use this website, we'll assume that you're OK with this. Dismiss