dezzy435
113400
2285
42
My friend is a math teacher. I thought this was hilariously and petty.
Nov 16, 2017 11:39 PM
dezzy435
113400
2285
42
My friend is a math teacher. I thought this was hilariously and petty.
JustANoobOnImgurWithOneCat
I think it would stay the same, however real life logic says it would slow down. So IDK
HatredandVitriol
A=πr², dA/dt=2πr(dr/dt), when r = 6 and dr/dt=3, then dA/dt=2π(6)(3)=36π cm²/h
hman1234
Basic related rates
IOnceAteASandwichAndTurnedIntoACanadian
The text being bent is distracting me from this problem
scarscantstopme
That stain is already dry around the edges, would not expand much more than it already has
silentsuspekts87
0.05mm/min.
HatredandVitriol
Someone try to do it without calculus.
APieceOfFuckingShit
I do not believe that is possible.
DavidNightingale
BuddyAboot
swagadelic1
I fucking hated related rates
APieceOfFuckingShit
MrRandom314159
Area = πr^2. Derivative (rate of change) of Area = 2πr*dr. r=6,dr=3. Therefore, The answer is 2*6*3*π= 36π(cm^2/hr)
PascalCasedUserName
Should give the test to the art teacher.
pourmacncheeseovermynakedbody
2?
eroso
Someone didn't read the question properly. Classic.
dargrin
get that access point down.
TheredBeImgurians
Isn't called "surfin' the internet" for nothing.
Smayds
Man, the 90s were so cool.
anewshineything
More importantly: sit under it, wait for it to fall on you, sue school.
eroso
Happened in our school. Water pipe had broken on the top floors. One tile fell on a students desk. The school got closed, 2 years of repairs
MightyIink
Idk, let me look it up the answer on the internet... ahhh the wifi is down!!!
DiracsDelta
Add in that the larger the stain gets, it affects the rate of spread and you get a differential equation.
eroso
Not in this case. That is not mentioned in the given information. Stuff like this is why I hated school.
FlowVector
Ooh that's a slippery slope! One day it's "how big will the circle be" and the next it's:
FlowVector
Solve the "system of linear second order constant coefficient non-homogeneous differential equations" or SLSOCCENHDE's for short...
MackdomblesDarbleChaiseBarga
Ayyy, I'm learning these right now. Just finished Laplace transform.
FlowVector
You're ahead of us. I'm doing it too, doing systems right now. I think elliptic curves and Laplace are next. Good luck on finals!
MackdomblesDarbleChaiseBarga
Thanks man, you too!
MousseCakePie
Shit just got exponential
FlowVector
Took me a second. God damn it +1
astrangehop
Of course as the area increases, the evaporation will slow down the spread. What kind of maths do you need for that?
FlowVector
I think that just makes it a multivariate problem
InterestingFacts
This is unrealistic. The area of the stain should increase at a constant rate, thus you should calculate for the rate of increase of radius
Morturis
Constant should be the amount of water /time, which would result in diminishing increase of the area
InterestingFacts
You are correct in your first statement, but I fail to see why this means diminishing increase in area? Are they not 1-1 correlated?
Morturis
I was wrong, they are indeed proportional, I was thinking of the radius. However you could say that greater surface means more evaporation
88wut
“Too fast. So call the custodian already.”
Felderburg
The stain seems close to some electronics; maybe that will give someone an incentive to fix it.
Jboogie1004
To
DirtyRotter
Its so hilariously
Arrowpuppet
Wouldn’t it just be 3cm/hr? Or does the current radius affect the speed?
[deleted]
[deleted]
PascalCasedUserName
Geometrically. The variable is in the base, not the exponent.
Hilaritybomber
It asks for the rate at which the area is increasing, not the radius.
NeoGenus59
dAdt = drdt 2 r pi = 3x 2 x r x 3.14 = i forgot the radius already
SpikeThruTheStreetRoof
Related rates , man
Talligan
A = pi()*r^2 so area increases exponentially faster than radius
Geo80
The area should be increasing linearly, not the radius.
Geo80
And that's neglecting evaporation. More area = more evaporation. Eventually they will reach equilibrium and the circle will stop growing.
PhysicsPhren
3 cm/hr is dr/dt, the rate of change of the stain radius over time. To find the rate of stain area increase, we first take a derivative 1/?
PhysicsPhren
with respect to time of A: A = pi * r^2, dA/dt = 2*pi*r* dr/dt. Plugging in values: 2*pi* 6*3 = 36 * pi cm^2 / sec, or about 113 cm^2/sec
Morturis
Dude you went way overboard. Pi*(r+3)^2 - pi*(r)^2 is good enough for a single instant, which is what they are asking. 141.3 cm/hr
OstrichNuggets
141.3 cm2/hr isn't the rate of change at the given moment, it's the linear approximation between r=6 and r=9. At r=6, dA/dt is 36pi not 45pi
njkayak99
It's a related rate problem. You have to take the implicit derivative of the area equation and plug in the known values.
ThyCountess
moonmoonjupiter
This. Its asking for instantaneous rate of change for the area at r=6cm
TheBestRussian
See, I understand most of those words, but I still have no idea what you said. And yet I aced first year calculus...but that was forever ago
Arracor
"Implicit derivative" What?
FlowVector
You take the derivative worth respect to time of the equation A=πr^2. Since it is with respect to time and you know the area and radius>
FlowVector
Change with time, the chain rule shows that you get the rates of both from the equation, A' and r'. So taking the implicit derivative gives>
LlamaFrequency
dA/dr = dA/dr x dr/dt. dr/dt given at time (t) so calculate dA/dr and multiply
0de9f6c0b2
36π cm^2/hr
WhiteningMcClean
nerd
DarkNinja2462
Beat me to it
tuxedobob
Nope, don’t remember how to solve this.
n0rs
You lose points if you don't show your working.
[deleted]
[deleted]
eroso
Stuff like this is why I hated school math. You were always supposed to guess what type of laws of physics and logic the teacher ment.
eroso
I still remember how in HS our teacher asked me to teach her the equations I used at an exam. I felt dissapointed and embarrased for her.
onethirdofakind
The RADIUS is increasing at 3 cm/hr, not ares. Calculus is needed for this.
eroso
You didn't read the follow up. Teacher decides what is wrong and what is right. In my HS, you always had to assume dumb.
onethirdofakind
*area No clue why autocorrect didn’t catch that.
MightAsWellBeCanadian
I thank you as a calculus teacher.
Morturis
You don't. The answer is incorrect. He calculated the area, no the speed at which it increases
139mod70
dr/dt = 3cm/h; A = pi*r^2; dA/dt = 2pi*r*dr = 2pi*r*3 = 6pi*r; dA/dt = 36pi @ t=6
FlowVector
That's a rate, and it's correct. You can see as the units are per hour
chickenoodlesoup
I used to have to have an umbrella over my monitor at work when was raining. Maintenance's response when it was raining, "Can't look at it
chickenoodlesoup
now because it's raining." Their response when it wasn't raining, "can't tell where the water was coming from because it's not raining."
MightyZephyr
We're doing our best- maintenance
SteevyT
My favorite one so far is "Hey, this 480V transformer is sitting in a puddle." "Eh, it's been fine for 40 years."
jtl3
I shudder at the tarps and wacky sheetmetal over some MCCs and transformers I see
spiffylookingnerfherder
Definitely don't go have a look when it's raining.
SteevyT
Don't go have a look? I have to walk right by it to get to my desk.
Xyzair
Wha? It's my desk, where else am I supposed to sit? /s
theshinobi23
Maintenance mechanic here. Gotta bug the hell out of us til we fix it just to get you to shut up.
Nephus
And that's what you hire facility managers for. To constantly bitch until you fix the thing.
BiliboyJenkins
My dad used to work plant shutdowns. He and a small crew would work through a 5+ year backlog of maintenance requests in 3 months.
BiliboyJenkins
He'd get yelled at by plant personnel for making them look bad.
chickenoodlesoup
They eventually balanced a bucket up there & told me to let them know if it overflowed or came crashing through the ceiling tiles.
castlemacaw