Maths question help please integration

13

Hi, please could I have some help on question 7.
For part A, I don’t know where I’ve gone wrong, I keep checking it but can’t find my mistake (sorry for my handwriting)
Question: https://ibb.co/Nn1pPkNy
My answer: https://ibb.co/HDhx7tX5
And then part b, I have no clue what it is asking and how to work it out.
Thank you!

Reply 1

Muttley79

20

Original post

by anonymous56754

Hi, please could I have some help on question 7.
For part A, I don’t know where I’ve gone wrong, I keep checking it but can’t find my mistake (sorry for my handwriting)
Question: https://ibb.co/Nn1pPkNy
My answer: https://ibb.co/HDhx7tX5
And then part b, I have no clue what it is asking and how to work it out.
Thank you!

When you integrate (something) ^ -0.5 why is the 'new' power still the same or have I misread - very messy?

Reply 2

mqb2766

21

Original post

by anonymous56754

Hi, please could I have some help on question 7.
For part A, I don’t know where I’ve gone wrong, I keep checking it but can’t find my mistake (sorry for my handwriting)
Question: https://ibb.co/Nn1pPkNy
My answer: https://ibb.co/HDhx7tX5
And then part b, I have no clue what it is asking and how to work it out.
Thank you!

The 5 should be 9 when you evaluate (4x+1)^(1/2) for x=20.
Note you could have simply done the integral of the curve, then subtracted 20sqrt(3)/9 at the end as its a trivial area of a rectangle.

For b, you should have done revolution about the x axis.

Reply 3

anonymous56754

OP

13

Original post

by mqb2766

The 5 should be 9 when you evaluate (4x+1)^(1/2) for x=20.
Note you could have simply done the integral of the curve, then subtracted 20sqrt(3)/9 at the end as its a trivial area of a rectangle.
For b, you should have done revolution about the x axis.

Oh yes, that was a silly mistake, thanks! I don't think revolution about the x axis is in my spec anymore so I can just ignore that.

Reply 4

mqb2766

21

Original post

by anonymous56754

Oh yes, that was a silly mistake, thanks! I don't think revolution about the x axis is in my spec anymore so I can just ignore that.

For the part a), it should be a relatively straight forward integral and while everyone makes mistakes, you were not able to spot the mistake. I guess a few things

•

As in the previous post, simply subtract a rectangle for the white area. Dont include it in the integrand as it simply adds extra/unnecessary working. In fact, when you evaluate the integral first, you miss off the x in that term, though it appears on the next line.

•

You did the integral by reverse chain. Its in the syllabus so a valid method, but Id have been tempted to make it clearer using a simple substitution so
u = 4x+1
dx = du/4
limits from 1 to 81
The integrand then becomes u^(-1/2) / 4 which is simple to evaluate and check. Its would be shorter than what you did and much clearer.

•

If youve made typo mistakes like this, think hard about how to avoid them in your a levels, otherwise it can eat time.

(edited 9 months ago)

Reply 5

anonymous56754

OP

13

Original post

by mqb2766

For the part a), it should be a relatively straight forward integral and while everyone makes mistakes, you were not able to spot the mistake. I guess a few things

•

As in the previous post, simply subtract a rectangle for the white area. Dont include it in the integrand as it simply adds extra/unnecessary working. In fact, when you evaluate the integral first, you miss off the x in that term, though it appears on the next line.

•

You did the integral by reverse chain. Its in the syllabus so a valid method, but Id have been tempted to make it clearer using a simple substitution so
u = 4x+1
dx = du/4
limits from 1 to 81
The integrand then becomes u^(-1/2) / 4 which is simple to evaluate and check. Its would be shorter than what you did and much clearer.

•

If youve made typo mistakes like this, think hard about how to avoid them in your a levels, otherwise it can eat time.

Thanks for the advice!

Reply 6

Kygoo

10

Original post

by anonymous56754

Hi, please could I have some help on question 7.
For part A, I don’t know where I’ve gone wrong, I keep checking it but can’t find my mistake (sorry for my handwriting)
Question: https://ibb.co/Nn1pPkNy
My answer: https://ibb.co/HDhx7tX5
And then part b, I have no clue what it is asking and how to work it out.
Thank you!

Hey...are you doing A levels Maths....what are your components?
I am also doing it and I have pure mathematics 1 and M1
Wanted a study budyy u good for it?

Maths question help please integration

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