Question

Pancake batter is poured into a pan to form a circular pancake who area increases at the rate of 3 cm^2/sec, so how fast is the radius increasing when the diameter of the pancake is 10 cm?

Answer 1

A = pi r^2
take the derivative of that first :)

Answer 2

one sec jiggle.

Answer 3

2(pi)(r)

Answer 4

be careful to take chain rule :)
like when you have r^2
derivative of r^2 = 2r (dr/dt) or 2r r'

Answer 5

so, its 2r?

Answer 6

derivative of A = pi r^2
would be A' = 2 pi r (r')
^ that's the part you always seem to be missing :)

Answer 7

4pi(r)!

Answer 8

erm
my last comment was a statement...
derivative of A
is A' = 2 pi r (r')

Answer 9

oh, so what's next?

Answer 10

I've gtg
so given A' = 3
d=10 so r = 5
plug in so 3 = 2 pi (5) (r')
solve for r'

Answer 11

oh, im confused

Answer 12

@agent0smith

Answer 13

The 2nd last line of @jigglypuff314's post has it all spelled out for you.

Answer 14

i can't do 3 = 2pi(5)(r') im shaking im so nervous

Answer 15

I don't understand why not... it's easy. Divide both sides by (2pi*5)

Answer 16

3/2pi^5?

Answer 17

I think it's 2pi*5 not 2pi^5...

3/(2pi*5)

Answer 18

3/10pi?

Answer 19

wow, you're just like her. thanks. got szome more