# Math Help - Maths B Stuck

1. ## Maths B Stuck

The volume of a cube increases at a constant rate of 10cm^3 per second . Find the rate of change in its total surface area at the instant when the sides are 20cm long.

I know that rate of change means I have to involve derivatives. The surface area formula for a cube = the length of a side say (x^2) x6 and the volume of a cube is x^2 x height .

I don't know how to form an equation in terms of time.

2. ## Re: Maths B Stuck

The side lengths are increasing over time, so x is a function of t.

To start, I always find it helps to write equations for everything you do know, and also to write down in mathematical terms what it is you are trying to find out. Can you do this?

3. ## Re: Maths B Stuck

What i do know is the formulas for surface area and volume of a cube. I know that the 10cm^3 increase per second would mean a certain distance is added to the lengths and height to equal 10cm^3 .

4. ## Re: Maths B Stuck

You did not understand what I wrote. Actually write down this information as EQUATIONS. It is essential!

5. ## Re: Maths B Stuck

I can't form the equuations . Every second the volume of the cube is increasing by 10cm^3 . I'm guessing something along the lines of V (t)= t 10x^3 but that wouldn't be it

6. ## Re: Maths B Stuck

Surely you can at least write the surface area and volume equations...