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Original post by leoishush

I think I am quite close to the answer but for some reason I just cant convert it to the right form, please someone help

Can't follow that at all - which part is this?

Original post by leoishush

I think I am quite close to the answer but for some reason I just cant convert it to the right form, please someone help

You're trying to find where dV/dt= 0. If you look at what you got when you differentiate and think about setting it to 0, it's natural to rewrite it as f(t) = g(t) for two fairly obvious functions f,g. Then think about how you can reduce one side to just be t.

(edited 1 year ago)

Original post by leoishush

I think I am quite close to the answer but for some reason I just cant convert it to the right form, please someone help

After your first two lines of working, you need to set dv/dt = 0, then rearrange to arrive at the given expression.

I can't follow what you've done, but when you get to the 2nd line (correct) I would advise setting that to 0 and multiplying by t+1 to clear the horrible fraction. Then rearrange from there.

Original post by davros

I can't follow what you've done, but when you get to the 2nd line (correct) I would advise setting that to 0 and multiplying by t+1 to clear the horrible fraction. Then rearrange from there.

Still need help?

Original post by mathsstudent222

let me know if u cant followit

Please observe the forum posting guidelines (sticky) - hints not full solutions - and remove your full solution.

Hi everyone thank you so much for your responses! I know a lot of you are confused as to what I have done after the second line. Here's what I did:

- 3rd line: divided everything by 0.4 ln (t+1)

- 4th line: factored out 0.5 in the fraction to get (25-t) in the numerator

- 5th line : cancelled out the 0.4

Many people have said to equal the equation to 0- I have already done that I just didn't make that obvious on the whiteboard (my bad)

This is for part b btw and I am really struggling to rearrange the equation for the required form

- 3rd line: divided everything by 0.4 ln (t+1)

- 4th line: factored out 0.5 in the fraction to get (25-t) in the numerator

- 5th line : cancelled out the 0.4

Many people have said to equal the equation to 0- I have already done that I just didn't make that obvious on the whiteboard (my bad)

This is for part b btw and I am really struggling to rearrange the equation for the required form

Original post by mathsstudent222

I posted solution but daddy moderator deleted it

ahhhhh oh well. I have seen the solution to it on youtube but I was wondering if I would get anywhere with the way I have done it ? I don't understand where I have went wrong

Original post by leoishush

ahhhhh oh well. I have seen the solution to it on youtube but I was wondering if I would get anywhere with the way I have done it ? I don't understand where I have went wrong

You're not necessarily wrong, but you have headed off in a slightly strange direction. What I would suggest is:

* Go back to line 2

* Start the line with "0 =" rather than just "="

* Concentrate on making "t" the subject of the equation

Original post by leoishush

ahhhhh oh well. I have seen the solution to it on youtube but I was wondering if I would get anywhere with the way I have done it ? I don't understand where I have went wrong

If I was doing it I would have worked back from the answer 1 step and tried to prove

(t+1) (1 + ln(t+1)) = 26

You dont seem to set dv/dt to zero. If you did youd get

0.4 ln(t+1) = (10 - 0.4t) / (t+1)

It should be straightforward to get the above answer by multiplying through and rearranging?

Original post by mqb2766

If I was doing it I would have worked back from the answer 1 step and tried to prove

(t+1) (1 + ln(t+1)) = 26

You dont seem to set dv/dt to zero. If you did youd get

0.4 ln(t+1) = (10 - 0.4t) / (t+1)

It should be straightforward to get the above answer by multiplying through and rearrangin

(t+1) (1 + ln(t+1)) = 26

You dont seem to set dv/dt to zero. If you did youd get

0.4 ln(t+1) = (10 - 0.4t) / (t+1)

It should be straightforward to get the above answer by multiplying through and rearrangin

ahhhh right. I have now tried doing that but for some reason i am still not getting the right answer

(line 3 I cancelled out the 0.4, line 4 I swapped t+1and ln(t+1)

Original post by mqb2766

I would have multiplied through, but add zero = 1 - 1 to the numerator should get you about there?

sorry i am confused

Original post by leoishush

sorry i am confused

Multiply through and add 1 - 1 to the right gives

(t+1) ln(t+1) = 26 - t - 1

....

Original post by mqb2766

Multiply through and add 1 - 1 to the right gives

(t+1) ln(t+1) = 26 - t - 1

....

(t+1) ln(t+1) = 26 - t - 1

....

for some reason I am still stuck with 25 in my working

Original post by leoishush

for some reason I am still stuck with 25 in my working

Multiply through you get

(t+1)ln(t+1) = 25 - t

you want 26, you also want a factor t+1 on the right. So add 0 = 1 - 1 to the right to get

(t+1)ln(t+1) = 25 + 1 - t - 1 = 26 - (t+1)

take the (t+1) on the right over, factorise and divide ...

For me there were a few ways to proceed but to rearrange the desired relationship as in the previous post, then notice that

10/0.4 = 25

and we needed 26 and an extra t+1 factor pretty much got it done.

(edited 1 year ago)

Original post by mqb2766

Multiply through you get

(t+1)ln(t+1) = 25 - t

you want 26, you also want a factor t+1 on the right. So add 0 = 1 - 1 to the right to get

(t+1)ln(t+1) = 25 + 1 - t - 1 = 26 - (t+1)

take the (t+1) on the right over, factorise and divide ...

For me there were a few ways to proceed but to rearrange the desired relationship as in the previous post, then notice that

10/0.4 = 25

and we needed 26 and an extra t+1 factor pretty much got it done.

(t+1)ln(t+1) = 25 - t

you want 26, you also want a factor t+1 on the right. So add 0 = 1 - 1 to the right to get

(t+1)ln(t+1) = 25 + 1 - t - 1 = 26 - (t+1)

take the (t+1) on the right over, factorise and divide ...

For me there were a few ways to proceed but to rearrange the desired relationship as in the previous post, then notice that

10/0.4 = 25

and we needed 26 and an extra t+1 factor pretty much got it done.

ohhhhhhhh thank you so much!

Original post by leoishush

ohhhhhhhh thank you so much!

For questions like this, if necessary try and work back a step or two. As there was t on both sides of the desired relationship, for me it indicated that multiplying through back from the final solution and just think what bits you have and how to rearrange / factorise them was important before really putting pen to paper.

(edited 1 year ago)

Original post by mqb2766

For questions like this, if necessary try and work back a step or two. As there was t on both sides of the desired relationship, for me it indicated that multiplying through back from the final solution and just think what bits you have and how to rearrange / factorise them was important before really putting pen to paper.

Ok thank you!

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