Question

determine the equation of a line perpendicular to this equation, at its y-intercept.

Answer 1

\[\frac{ 1 }{ 2 }e ^{x+1}\]

Answer 2

Did you find the y intercept?

Answer 3

yes, i got e/2

Answer 4

so my point is (0, e/2). My derivative is \[\frac{ 1 }{ 2 }xe ^{x+1}\]. Would I sub in 0 for the equation to solve for the slope? because I get 0 as a slope and the back has 2/e

Answer 5

You want the slope at the y intercept which is e/2

Answer 6

yes, so would I sub that in instead

Answer 7

Oh. That's right...you need the slope at the x value 0.

Answer 8

but the slope at x value at 0 is 0

Answer 9

The first derivative is the same as the original function.

Answer 10

So if you replace x with 0 you do get e/2 for the slope.

Answer 11

no, my first derivative is \frac{ 1 }{ 2 }xe ^{x+1}

Answer 12

no

Answer 13

\[\frac{ 1 }{ 2 }xe ^{x+1} \]

Answer 14

OMG I didn't take the derivative of x

Answer 15

\[\frac{ 1 }{ 2 }e ^{x+1}\]

Answer 16

that's it. and if you replace x with 0 you get e/2 for the slope

Answer 17

yes, ugh so stupid. Can you help with another question

Answer 18

Maybe...I don't know a lot.

Answer 19

Radium decays at a rate that is proportional to its mass, and has a half-life of 1590 years. If 20g of radium is present initially, how long will it take for it to decay to 90% of its its mass.

Answer 20

I tried to do it, but I got a different answer

Answer 21

Remind me of the exponential decay formula.

Answer 22

Mf=Mi(1/2)t/h

Answer 23

^t/h

Answer 24

\[18=20(\frac{1}{2})^{]\frac{t}{1590}}\]

Answer 25

Is that what you did?

Answer 26

yes

Answer 27

\[\ln .9=\frac{t}{1590}\ln .5\]

Answer 28

yes

Answer 29

\[t=\frac{\ln .9}{\ln .5}\times 1590=241.7\]

Answer 30

Is that what you got?

Answer 31

yes

Answer 32

but its not the answer my book gives

Answer 33

And what is the real answer?

Answer 34

5282 years

Answer 35

I didn't remember that formula. Let me research the formula.

Answer 36

ok

Answer 37

Still need help?

Answer 38

yes

Answer 39

\[A=A _{0}e ^{kt}\]

Answer 40

Use that to find the constant.

Answer 41

\[50=100e ^{k(1590)}\]

Answer 42

\[\ln .5=1590k\]

Answer 43

\[k=-.0004359\]

Answer 44

could you explain why I need the constant, because its the idea behind the question I don't get

Answer 45

\[18=20e ^{-.0004359t}\]

Answer 46

Because that is the exponential decay equation. They gave you the half-life so you could find the constant of variation. Remember the problem said it decays at a rate proportional to its mass.

Answer 47

so how will I know when to do it for another question

Answer 48

I guess when you don't know the constant.

Answer 49

stupid question, but how will I know what the constant is?

Answer 50

and if it gives me one, I know how to find it now, but I don't get how to know if i'm given one or not

Answer 51

If you are given the constant, it will be identified as such.

Answer 52

ok. umm could you help me with a vector question

Answer 53

You might want to post it as a new question. I am not good with vectors.

Answer 54

i did, but no one answered

Answer 55

could I ask you a question unrelated to math

Answer 56

yes

Answer 57

Where is the vector question. I can look at it and see if I know how to do it.

Answer 58

I have exams in a week or so. I have so much anxiety right now, that I shake when studying. I know everything, but I fear that on my exam there will be a question that I will know what to do, but I won't be able to do it. Because this has happened before and I spend the whole class working on one question and left 4 other question blank. I don't know what to do.

Answer 59

http://openstudy.com/users/0213#/updates/51b661d2e4b06ee3ee28a37d

Answer 60

Go through the test and do all the problems you know how to do. The ones you are not sure of..go back to at the end starting with the one you are most familiar with.

Answer 61

what about until then, bc i'm freaking out about studying

Answer 62

Do you have a study guide?

Answer 63

no, just my notes, and my teacher keeps saying the exam is a killer and that if ppl study the will know what to do, but its a matter of knowing how to do it. Like I could know how to take the derivative of a ln function, but I don't know were to start.

Answer 64

its just not sitting well with me, since I study for his tests and then mess up

Answer 65

Why don't you repost that other question. It is closed. If you keep reposting it, eventually someone will answer it. I don't know how to do it.

Answer 66

its ok, i will ask my teacher

Answer 67

Are you trying to memorize how to do the problems? In math, you have to know what to memorize and what to reason out.

Answer 68

no, like I know the idea behind the equations, chain rule, product rule etc. and I know how to apply them. I memorize the log properties, but not the questions.

Answer 69

You will be fine. Just relax and do your best...that's all anyone can expect.

Answer 70

but my best is in the 70s which is a B- to a B+. I want to get at least a A- for my university to see.