The Student Room Group

Integration with parametic equation

I need help please on question 15)d)

Scroll to see replies

Reply 1
Original post by otrivine
I need help please on question 15)d)


What have you done so far? I'd start by finding the area of the right-angled triangle you've drawn on the figure and subtract from it the area under the curve.
Reply 2
Original post by davros
What have you done so far? I'd start by finding the area of the right-angled triangle you've drawn on the figure and subtract from it the area under the curve.


I integrated 4sinx which is -4cosx+c ? assume x is detha,

But I dont get the limits?
Reply 3
Original post by otrivine
I need help please on question 15)d)

Firstly, it's unclear from the diagram but I think that the required area is just the lower bit that looks like a triangle - not the upper bit with the ink lines on.
There's a neat way and a hackish way. You can find y in terms of x, by finding theta in terms of x and substituting. Alternatively, and much nicer, you can use the formula:
ydx=ydxdθdθ\displaystyle \int y dx = \int y \frac{dx}{d \theta} d\theta.
Reply 4
Original post by Smaug123
Firstly, it's unclear from the diagram but I think that the required area is just the lower bit that looks like a triangle - not the upper bit with the ink lines on.
There's a neat way and a hackish way. You can find y in terms of x, by finding theta in terms of x and substituting. Alternatively, and much nicer, you can use the formula:
ydx=ydxdθdθ\displaystyle \int y dx = \int y \frac{dx}{d \theta} d\theta.


No I used the area formula
Reply 5
Original post by otrivine
No I used the area formula

Ah, if there's a formula that spits out what you need then feel free to ignore me :smile:
Reply 6
Original post by Smaug123
Ah, if there's a formula that spits out what you need then feel free to ignore me :smile:


But dont get how they did the limit?
Reply 7
Original post by otrivine
No I used the area formula


What area formula

Smaug is correct

You found the limits when you found the co-ordinates of P and R
Why don't you create a triangle with the first quadrant inside it. Then you can take away the area of the triangle away from the area of the curve from the triangle.
Reply 9
Original post by TenOfThem
What area formula

Smaug is correct

You found the limits when you found the co-ordinates of P and R


Because to find the area for intgration , you integrate y with the limits
Original post by otrivine
Because to find the area for intgration , you integrate y with the limits


Yes, as Smaug said
Reply 11
Original post by TenOfThem
Yes, as Smaug said


and so you integrate only 4sinx
Reply 12
Original post by otrivine
Because to find the area for intgration , you integrate y with the limits


You integrate y(x) with respect to the x-limits. If you integrate with respect to a parameter then you must ensure that your limits are the appropriate values of the parameter.
Reply 13
Original post by otrivine
Because to find the area for intgration , you integrate y with the limits

Can you give us the expression you're integrating, so we know what you mean by "area formula"? Then we might be able to help with the limits…
Original post by otrivine
and so you integrate only 4sinx


You need to integrate y with respect to x

but y = 2Sin(theta) so you need to do what Smaug said to do
Reply 15
this is what I mean?
Reply 16
Original post by otrivine
this is what I mean?


The limits are the theta limits, and you need to rewrite your dx in terms of d(theta) as smaug showed you earlier.
Original post by otrivine
this is what I mean?


That is incorrect

You can not integrate Sin(theta) with respect to x

You need to use the formula that Smaug showed you



The limits are

x/theta at point P
x/theta when y=0
Reply 18
Original post by otrivine
this is what I mean?

You haven't actually integrated correctly. You've integrated d(theta) instead of d(x).
Reply 19
Original post by Smaug123
You haven't actually integrated correctly. You've integrated d(theta) instead of d(x).


ok :smile:

what would you integrate then can you show me:redface:

Quick Reply

Latest