Answer:
Step-by-step explanation:
This is a pretty basic related rates problem. I'm going to go through this just like I do in class when I'm teaching it to my students.
We see we have a snowball, which is a sphere. We are talking about the surface area of this sphere which has a formula of
In the problem we are given diameter, not radius. What we know about the relationship between a radius and a diameter is that
d = 2r so
Now we can have the equation in terms of diameter instead of radius. Rewriting:
which simplifies to
and a bit more to
(the 4's cancel out by division). Now that is a simple equation for which we have to find the derivative with respect to time.
Now let's look at the problem and see what we are given as far as information.
The rate at which the surface area changes is -3.8, and we are looking for 
, the rate at which the diameter is changing, when the diameter is 13. Filling in:
and solving for the rate at which the diameter is changing:
and divide to get
Obviously, the negative means that the diameter is decreasing.
All you have to do is multiply 18.3 x 5.1
18.3×5.1=93.33
It will take 93.33 minutes to repair the entire section. Hope that helps!
In that situation, you use the information you DO have. It's not always
enough to do the job. But often it is, and in that case, you use what you
do have to answer the question or solve the problem.
Sadly, I can't be any more specific than that. You haven't told us what
information you do have ... only what information you don't have.
