One for the maths geniuses - Double integration

[K87]

Racking my brains with this one guys, can anyone help out? I have an anser but just tried it through a computer programme and its come out different.

((xy) + (4/y)) dy dx

x limits of 0 and 4
y limits of 2 and 5

Answers on a postcard.

[lightningghost]

God, it's several months since I did anything like this. I feel like I need an equation to do it, then it would be pretty straight forward.


Arse.

[central]

<runs away, screaming>



1981 A level drop-out.

[K87]

haha its part of my coursework for a uni module. Dreading the exam for it!

[remedy]

Eleventy three





















... No, my mistake, I carried the 4(x) - twelvety eight.

[LadyB]

I started working this out on the calculator but got bored .... I'm now using a screwdriver on the calculator to see where the numbers live; far more interesting

[blank]

121?

Scratch that - 98.7.

Do you want a solution or just the answer?

Edited by blank on Friday 11th December 23:22

[K87]

hmm I got 90

[MoleVision]

K87 said:

hmm I got 90

2566.512...


I may have forgotten how to do this (I did get a B at A level further maths, honest!!)

Hope you never get to see a complex fourier transform.

2nd attempt I get 118.02.. however this is also very likely going to be wrong! Wish my graphic calculator had some batteries in it.

Edited by MoleVision on Friday 11th December 23:34

[blank]

Definitely 98.7. Confirmed by hand and MATLAB. (first attempt I added 11 instead of taking it away, hence being 22 out)

[Zod]

I used to be able to do this 24 years ago.

[TVR MAN]

blank said:

Definitely 98.7. Confirmed by hand and MATLAB. (first attempt I added 11 instead of taking it away, hence being 22 out)

Agreed.

Why don't you post up some of your working and we can diagnose it if you like?

[K87]

ok so integrating first with respect to y I get this:

[(xy^2)/2 + 4log(y)] dx

Applying the y limits gives:

((25x/2) + 2.7959) - ((4x/2) + 1.2041)


= ((21x/2) + 1.5918) dx

Then integrating with respect to x I get:

[(10.5x^2)/2 + (1.5918x)] and applying x as 4 gives

84 + 6.3672 = 90.3672

[SlipStream77]

You need a copy of this...

http://www.amazon.co.uk/Engineering-Mathematics-6t...

[MoleVision]

TVR MAN said:

blank said:

Definitely 98.7. Confirmed by hand and MATLAB. (first attempt I added 11 instead of taking it away, hence being 22 out)

Agreed.

Why don't you post up some of your working and we can diagnose it if you like?

right.. got it... 98.4 was my ans but there may have been some rounding in there.

working...

nb:
{}denote limits
[] denote integrated funcitons that have not had limits applied

split into INT(xy)dydx + INT(4/y)dydx

deal with INT(xy)dydx 1st.

INT{0-4} (INT{2-5}xydy) dx
is
INT{0-4} [x(y^2)/2]{2-5} dx
is
INT{0-4} (10.5x) dx
is
5.25(x^2){0-4}
which equals 84.

same for other half

INT{0-4} (INT{2-5}(4/y)dy) dx
is
INT{0-4} [4*lny]{2-5} dx
is
INT{0-4} (3.6) dx
is
3.6x{0-4}
which equals 14.4

add 14.4 to 84 to get 98.4.

I always made daft mistakes in maths exams so no doubt that will account for the missing 0.3 I appear to have over the above answers.
Hope that make an iota of sense.. and is correct.

Edited by MoleVision on Saturday 12th December 00:01

[K87]

MoleVision said:

K87 said:

hmm I got 90

Hope you never get to see a complex fourier transform.

Edited by MoleVision on Friday 11th December 23:34

Thats the second half of the coursework!

[MoleVision]

K87 said:

MoleVision said:

K87 said:

hmm I got 90

Hope you never get to see a complex fourier transform.

Edited by MoleVision on Friday 11th December 23:34

Thats the second half of the coursework!

Then all I can do is to wish you all the best

Mech Eng by any chance?

[K87]

SlipStream77 said:

You need a copy of this...

http://www.amazon.co.uk/Engineering-Mathematics-6t...

Sat right infront of me, although think its more useful for working out the biceps by the size of it!

[TVR MAN]

K87 said:

ok so integrating first with respect to y I get this:

[(xy^2)/2 + 4log(y)] dx

Applying the y limits gives:

((25x/2) + 2.7959) - ((4x/2) + 1.2041)


= ((21x/2) + 1.5918) dx

Then integrating with respect to x I get:

[(10.5x^2)/2 + (1.5918x)] and applying x as 4 gives

84 + 6.3672 = 90.3672

Ahhh.

This is a really simple problem to fix. Use the natural log button on your calculator which is the "ln" one.

You are using the "log" button which is different. When you integrate 1/x dx you get ln(x) which is log to the base e of x.

You should keep it written as 4ln5 etc until the final step and then you can say approximately what it is.

[K87]

Yup just worked that out too. So I should always use the Ln button not the log button for integration?