You are Here: Home >< Maths

# Differential Equation question - C4 watch

Announcements
1. Just working through my revision guide and I can't do this question.

~ At time t minutes an ink stain has area . When t=1, A=1, and the rate of increase of A is given by

a) Find an expression for A in terms of t.

I got

but I'm not sure if this is right, and I'm a bit unsure as to how you integrate

Thanks
2. (Original post by allyourbase)
Just working through my revision guide and I can't do this question.

~ At time t minutes an ink stain has area . When t=1, A=1, and the rate of increase of A is given by

a) Find an expression for A in terms of t.

I got

but I'm not sure if this is right, and I'm a bit unsure as to how you integrate

Thanks
You need to write it as a power.

Simplify, and integrate from there.

Quote me if you'd like any more help
3. well

1/(A square root A) = A ^ (-3/2) becausre square root of A is A^1/2,

beaten to it again lol
well

1/(A square root A) = A ^ (-3/2) becausre square root of A is A^1/2,

beaten to it again lol
5. (Original post by Muppety_Kid)
You need to write it as a power.

Simplify, and integrate from there.

Quote me if you'd like any more help
Ok, I get

Anyone see where I'm going wrong? Sorry for being crap at maths
6. (Original post by allyourbase)
Ok, I get

Anyone see where I'm going wrong? Sorry for being crap at maths
Ugh, you're doing Edexcel aren't you? Me too

Give me 5 minutes to work it through myself...
7. (Original post by allyourbase)
Ok, I get

Anyone see where I'm going wrong? Sorry for being crap at maths
That's right (except the minus sign you seem to have picked up). Multiply top and bottom by t^2.
8. (Original post by generalebriety)
That's right (except the minus sign you seem to have picked up). Multiply top and bottom by t^2.
Just doing it now. I see where the minus sign has come in - look at my 5th and 6th lines lol.
9. (Original post by allyourbase)

You see, where you halved both sides, you've taken instead of
Is that right?
10. (Original post by generalebriety)
That's right (except the minus sign you seem to have picked up). Multiply top and bottom by t^2.
Would you mind showing me how that works?

When I times top + bottom by I get

11. (Original post by allyourbase)
Would you mind showing me how that works?

When I times top + bottom by I get

That wasn't the line I wanted you to work with. Try multiplying the line below that by t^2/t^2.
12. (Original post by DevonHowell)
You see, where you halved both sides, you've taken instead of
Is that right?
Is that rhetoric? I'm genuinely confused

If you're asking because you're unsure yourself, it's in the spoiler below.

Spoiler:
Show
No, it's not right. It should be - he's forgotten to take out the - all the way through on the right hand side.
13. (Original post by Muppety_Kid)
Is that rhetoric? I'm genuinely confused

If you're asking because you're unsure yourself, it's in the spoiler below.

Spoiler:
Show
No, it's not right. It should be - he's forgotten to take out the - all the way through on the right hand side.
For some reason I'd changed a + to a - . Dunno why.. Just a mistake.
14. (Original post by generalebriety)

That wasn't the line I wanted you to work with. Try multiplying the line below that by t^2/t^2.
Yeh, it's a crafty trick, but sometimes they multiply the top and bottom by the same (so the fraction's in the same "ratio"). It's not strictly in its simplest form then, but it's a lot easier to work with than what you've got currently, especially if it asked you to find the area when t is a certain value.
15. (Original post by allyourbase)
For some reason I'd changed a + to a - . Dunno why.. Just a mistake.
Sorry, I wasn't having a go at you. The other guy asked if you were right when you did that, and I didn't know if he was trying to give you a hint, or he didn't understand.
16. I have tried to help by highlighting a possible mistake. I was not 100% and I'd written it before I saw someone had already mentioned the mistake.
17. (Original post by generalebriety)

That wasn't the line I wanted you to work with. Try multiplying the line below that by t^2/t^2.
****.

thanks <3 x23532
18. (Original post by DevonHowell)
I have tried to help by highlighting a possible mistake. I was not 100% and I'd written it before I saw someone had already mentioned the mistake.
I wasn't having a go at you either.

Both of you, please, DON'T HURT ME!
19. (Original post by allyourbase)
****.

thanks <3 x23532
For what it's worth:

, and so

.
20. (Original post by generalebriety)
For what it's worth:

, and so

.
I blame my maths teachers of course

Thanks

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: June 7, 2008
Today on TSR

And I hate it

### University open days

• Heriot-Watt University
School of Textiles and Design Undergraduate
Fri, 16 Nov '18
• University of Roehampton
Sat, 17 Nov '18
• Edge Hill University
Faculty of Health and Social Care Undergraduate
Sat, 17 Nov '18
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams