Question

A police car is located 60 feet to the side of a straight road.

A red car is driving along the road in the direction of the police car and is 130 feet up the road from the location of the police car.

The police radar reads that the distance between the police car and the red car is decreasing at a rate of 100 feet per second.


How fast is the red car actually traveling along the road?

Calc. 1 I used Pythagorean Theorem to get the hyp. Then pluged in the givens into the derivative of Pyth. theorem.

I drew this as a triangle where x direction = 60, y direction = 120. dL/dt = -100. Got L=143.18 from Pyth. Theorem. L = hypotenuse. L^2=X^2+y^2. d/dy(L^2=X^2+y^2) is 2L*dL/dt=2x*dx/dt + 2y*dy/dt. I put 0 in for dx/dt bacause the police car isn't moving, then solved. AHH idk

I keep getting the wrong answer. How would you guys do this?

Answer 1

Can you draw a simple picture to show how they actually look like? Sorry but I can't imagine it in my head.

Answer 2

How do I select a color to draw? It's coming out blank ah

Answer 3

using a new browser now ill try it out again

Answer 4

Is it like this?

Answer 5

That looks more complicated ah ill draw out what i did sec

Answer 6

A is actually where the car is, B is the point where the car will be after a short period of time.

Answer 7

Oh I see

Answer 8

b/c its 130 ft up the road so it wouldn't be the hyp. if that clears it up idk

Answer 9

Yes, I understand know.

Answer 10

The problem says that "the distance between the police....rate of 100 ft per sec". Therefore, 1 sec after, the distance will be L - 100

Answer 11

what do i do with the result of that ;o

Answer 12

Now you can calculate x

Answer 13

kk

Answer 14

Do i plug into derivative of the pyth theorem? or.... is that the incorrect way to do this

Answer 15

Yes. I believe you can do it. I forgot how to deal with that "d" thing. :)

Answer 16

i wrote it above but i dont know what to plug in for dx/dt
because the police car isn't moving so i put 0

Answer 17

ill try it out

Answer 18

Yes, I will try to remember those things.

Answer 19

keep the negative right?

Answer 20

i got -168.11 ft per sec thats sooooo fast o.o

Answer 21

I think. Oh and I think \(\sqrt{13^2+6^2}\approx 143.2\)

Answer 22

i couldn't even get that right on the calculator LOL wow

Answer 23

130 and 60 sorry

Answer 24

sec let me try this again from the beginning XD

Answer 25

Sorry that I can't help. I forgot everything about derivative

Answer 26

np i appreciate the help

Answer 27

the hypotenuse will be 43.2 after 1 second right?

Answer 28

Hum, it can't be because the side is greater than the hypotenuse

Answer 29

yeah i was about to say that ;/

Answer 30

My teacher didn't go over this type of question in class, i dont have anything to refer to and this is the last question T_T

Answer 31

do you know any other homework sites?

Answer 32

I can't think of one. Moreover, this site supposes to be the best. I don't understand why the professionals didn't come and help

Answer 33

And I haven't reach that yet. We are in trig

Answer 34

is there a way to do call outs for help?
like "@Namehere"

Answer 35

nvm @ worked but i dont know which names to choose

Answer 36

Yes

Answer 37

Just click on the Math, a list of pros will come up

Answer 38

One more thing, why don't we try 0.5s instead of 1s?

Answer 39

ill give that a try

Answer 40

-130.682 ft/sec

Answer 41

idk T_T

Answer 42

I believe in you, and you should do so

Answer 43

@Preetha

Answer 44

Is that the way to call out ppl?

Answer 45

@aum

Answer 46

Yes. @Preetha Would you mind checking his work?

Answer 47

@jim_thompson5910

Answer 48

I'm just going to enter that. I'll get diff numbers if its wrong but its better than sitting around

Answer 49

Yeah, believe in yourself.

Answer 50

wrong ;/

Answer 51

i dont know what to plug in for dx/dt and its for sure not 0 since i keep getting wrong answers