velox
Retired
in the attached picture, how do they get dr/dt = 2
I dont understand what they have done to get it.
I dont understand what they have done to get it.
-
Finished your HSC? Check out our University Discussion, Non-School forums or help out next year's HSC students! -
YOU can help the next generation of students in the community! Share your trial papers and notes on our Notes & Resources page
Rates and Change Calculus Problem (1 Viewer)
- Thread starter velox
- Start date
-=«MÄLÅÇhïtÊ»=-
Gender: MALE!!!
the only step that that solution doesnt show is
dA/dt=dA/dr*dR/dt
so basically u wanna find dA/dt
u know dR/dt=2 (given)
What's dA/dr?
Find A as function of r. In this case, its area(A)=pi*r^2
dA/dr=2pi*r
2pi*r * 2
=2*pi*10*2 (where r=10, given)
=40pi m^2/s (don't forget to put units with answer)
edit:
oops i realise now u only wanted to know why dR/dt=2.
You'll find that wiv these rates of change questions that you're usually given some data. just use it.
dA/dt=dA/dr*dR/dt
so basically u wanna find dA/dt
u know dR/dt=2 (given)
What's dA/dr?
Find A as function of r. In this case, its area(A)=pi*r^2
dA/dr=2pi*r
2pi*r * 2
=2*pi*10*2 (where r=10, given)
=40pi m^2/s (don't forget to put units with answer)
edit:
oops i realise now u only wanted to know why dR/dt=2.
You'll find that wiv these rates of change questions that you're usually given some data. just use it.
Last edited:
velox
Retired
so ignoring the ext 1 method, dr/dt = 2 how? I dont think its *just* given to you
It certainly is just given to you, as acmilan1987 said.
"Their radii increase at the rate of 2m/s."
a) You know that the radius is changing at a rate of +2 metres per second.
b) dR/dt is the rate at which R (the radius) is changing with respect to t (time).
Therefore dR/dt = 2.
It's important to understand what you're actually doing (i.e. what the rates and the notation 'dy/dx' truly mean) if you want to be able to access the top mark range in the exam. Rote-learning and memorisation just don't cut it. You want to understand.
"Their radii increase at the rate of 2m/s."
a) You know that the radius is changing at a rate of +2 metres per second.
b) dR/dt is the rate at which R (the radius) is changing with respect to t (time).
Therefore dR/dt = 2.
It's important to understand what you're actually doing (i.e. what the rates and the notation 'dy/dx' truly mean) if you want to be able to access the top mark range in the exam. Rote-learning and memorisation just don't cut it. You want to understand.