You are Here: Home

# P2 Differential Equation Help watch

1. Hey, can you check my working for this question please. :

"The volume of a sphere is increasing at a rate of 75cm^3/s. Find the rate at which the radius is increasing at the instant when the radius of the sphere is 15cm."

My working :

dv/dt = 75

v = 4/3*pi*r^3

dv/dr = 4*pi*r^2

dr/dt = dr/dv * dv/dt

Therefore dr/dt = 75/(4*pi*r^2)

When r = 15

dr/dt = 0.0265

Is this right? any comments will be appreciated. Thanks.
2. looks right to me (but i've not checked things like your muliplication)

good luck tomorrow!

lou xxx

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: June 20, 2004
Today on TSR

### Life lessons uni will teach you

Questionable decision-making and constant surprises

### University open days

1. Loughborough University
Fri, 21 Sep '18
2. University of Cambridge
Fri, 21 Sep '18
3. Richmond, The American International University in London