If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

2015 AP Calculus AB/BC 1ab

Water in a pipe.

Want to join the conversation?

• at , you calculated the answer in radians. Why did you use radians and how do you know when to use radians or degrees?
• When in doubt, assume radians. Almost all mathematicians use radians by default.

You can tell the difference between radians and degrees by looking for the `°` symbol. If the numbers of an angle measure are followed by a `°`, it will be degrees. Otherwise it will always be radians.

I hope this helps!
• For part b, since the d(t) and r(t) indicates the rate of flow, why can't we just calc r(3) - d(3) to see the whether the answer is positive or negative?
• Excellent. And a much better answer since it shows you can give an answer at any time t. Plus it's quicker, and easier to follow.
• In part one, wouldn't you need to account for the water blockage not letting water flow into the top because its already full?
• The blockage is already accounted for as it affects the rate at which it flows out. It does not specifically say that the top is blocked, it just says its blocked somewhere. That blockage just affects the rate the water comes out. That is why there are 2 different equations, I'm assuming the blockage is somewhere inside the pipe.
• In part A, why didn't you add the initial variable of 30 to your final answer?
(1 vote)
• its not asking for total amount, just how much flowed in
• What about the initial 30 cubic feet of water when t = 0 ?
• Units: I think you should have noted the units used in both parts to ensure that you didn't need to convert anywhere.
In these problems you were perhaps lucky that it didn't matter, in that the rate was cubic feet/hour, and time in hours. But they MIGHT have given the rate in cubic feet per MINUTE, or perhaps the time in minutes.
Sanity Check: You should ALWAYS try to do a rough check on your final answer if you've used some complex series of calculations, or some software (i.e. a calculator program). To make it simple, you could do three rough intervals perhaps, and time 0, 4 and 8. Calculate the rate at each, assume rate was proportional for each, and calculate the sums. Are easy calc's, just triangle and squares.
• How do you know when to put your calculator on radian mode?
(1 vote)
• Usually for AP calculus classes you can assume that your calculator needs to be in radian mode unless otherwise stated or if all of the angle measurements are in degrees. I don't think I can recall a time when I was asked to use degree mode in calc class, except for maybe with some problems involving finding lengths of sides using tangent, cosines and sine. I hope this helped!