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hello, i have a related rate problem with area. (attached below)

Mathematics
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i first started the problem like this: facts: altitude: increasing 2.000 A=4.000 altitude: increasing 11.500 A=82.000 so Area= (1/2) * base* height At what rate is the base of the triangle changing when the altitude is 11.500 centimeters and the area is 82.000 square centimeters? 82.000=(1/2)*base* 11.500 82.000=5.75b 14.2609=b ?? help
the derivative of 1/2bh?

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Other answers:

yes i know the product rule
Yep. Thats what these related rate problems require. If you have a change in area then you definitely need an area equation. And since the area is changing, you know its the derivative of the area. And yeah, the derivative of 1/2bh.
  • phi
if it helps, the area A, base B and height H are all functions of time (they are changing) A(t) = 0.5 * H(t) * B(t) take the derivative with respect to time
so not respect to b?
  • phi
the change in Area with respect to time is also known as the rate you want dA/dt , and so on..
I'm confused with finding the derivative.
alli know is this formula: a=pi^2 r but im guessing it's not relevant for this problem.
and also a=1/2*b*h
This is a triangle, though, so we don't even have pi. But yes, you would use 1/2bh since its the area of a triangle, which is what we have.
right
Now its just that every time you take the derivative of a variable, you also write dx/dt, dy/dt, etc along with it. These dx/dt things represent the changes given in your problem. So if you decided to make the altitude h, when you take the derivative of it, you would have dh/dt. This dh/dt is what gets replaced with 2.000cm/min value given. But if I'm confusing ya I'm sure phi can help.
  • phi
to take the derivative of A with respect to time you write a little d in front, and divide by dt
dA/dt
  • phi
yes that is the left side you have dA/dt = 0.5 * d/dt ( B * H) use the product rule on B*H
oh okay so we ignore the 0.5 for now?
  • phi
You don't ignore constants, but you can keep them on the outside of the derivative just like d/dt (2x) = 2 *dx/dt
so here: dA/dt=1/2(dh/dt)+(db/dt)
  • phi
except that is not the product rule http://www.1728.org/chainrul.htm
  • phi
the product rule of d/dt (x * y) = x * dy/dt + y * dx/dt
  • phi
?
@phi sorry connection went off
i understand what you're saying up there.
is this right? dA/dt= 1/2 (b*dh/dt)+(h*db/dt)
  • phi
is this right? dA/dt= 1/2 (b*dh/dt)+(h*db/dt) yes, but the 1/2 multiplies both terms \[ \frac{dA}{dt}= \frac{1}{2}\left( b\frac{dh}{dt} + h\frac{db}{dt} \right)\\ 2\frac{dA}{dt}= b\frac{dh}{dt} + h\frac{db}{dt} \] now fill in the various quantities and solve for db/dt

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