HSC 2012 MX1 Marathon #1 (archive) (1 Viewer)

[Carrotsticks]

Re: 2012 HSC MX1 Marathon

need help with this question
Find the volume of the solid generated by rotating each region about: x axis and y axis

x=y^2, x=4 shaded region between curve and x = 4.

i can find the volume of x but not y axis

PS: This is for white volume, not orange volume.

 

[Timske]

Re: 2012 HSC MX1 Marathon

hmmm i see what u did there i got 32pi/5 i guess according to ur working out i didnt times 2pi but the answer in the textbook says 128pi/5

 

[Peeik]

Re: 2012 HSC MX1 Marathon

perhaps the working out is....>>>

 

[bleakarcher]

Re: 2012 HSC MX1 Marathon

You so sleepy

 

[bleakarcher]

Re: 2012 HSC MX1 Marathon

hmmm i see what u did there i got 32pi/5 i guess according to ur working out i didnt times 2pi but the answer in the textbook says 128pi/5

It should be 2*128pi/5 the author should have said that the region was bounded by the x-axis if that answer was to be true.

 

[Timske]

Re: 2012 HSC MX1 Marathon

volume bounded by the x axis is 8pi piek how did u get 16 - y^4?

 

[Peeik]

Re: 2012 HSC MX1 Marathon

Come to think of it, i may of misread the question....is the volume bounded by the x axis for part b?? Because if it is....my answer is wrong.

(Difficult to explain here without a diagram but here it goes)

Well for my answer, i thought it was only looking in the positive y quadrant. So if you look at carrots picture, i thought u only needed the part in the first quadrant. If you rotate that part alone about the y axis, you should see that you cant do your normal volume integral. So what i did was i found the entire volume below the line x=4 (about the y axis) and subtracted it from the volume below the curve x=y^2. Then you square it because of the volume formula>> 4^2 - (y^2)^2.

Sorry if that was hard to understand i am a bit sleepy and its a shame i dont have a tablet so i can draw diagrams .

 

[Carrotsticks]

Re: 2012 HSC MX1 Marathon

Oh my, I forgot to subtract that answer from the cylinder! What am I thinking!? This indicates bed time.

 

[Timske]

Re: 2012 HSC MX1 Marathon

um carrot the answer is 128pi/5

i think u mean radius 4 and height 2

i see teh glory now

v = pi x 4^2 x 2 = 32pi

32pi - 32pi/5 = 128pi/5

 

[Peeik]

Re: 2012 HSC MX1 Marathon

im so lost

Perhaps it would make our life easier if you type the question out EXACTLY like it is written in the textbook. The author of that tbook may be wrong by the looks of it.

 

[Timske]

Re: 2012 HSC MX1 Marathon

um i did type the question out as it was written in the textbook. considering it is a graph

textbook is cambridge 3unit yr 11G question 8c.

 

[Carrotsticks]

Re: 2012 HSC MX1 Marathon

um carrot the answer is 128pi/5

i think u mean radius 4 and height 2

i see teh glory now

v = pi x 4^2 x 2 = 32pi

32pi - 32pi/5 = 128pi/5

If the question said "Area bounded by the line x=4, x=y^2 and the x axis", then 128pi/5 would be correct.

However, it does not, so we can't assume it.

As a result, our answer is twice the answer acquired by including the x axis in the bound.

 

[Timske]

Re: 2012 HSC MX1 Marathon

need help with this question
Find the volume of the solid generated by rotating each region about: x axis and y axis

x=y^2, x=4 shaded region between curve and x = 4.

i can find the volume of x but not y axis

its both x and y axis
volume at x axis is 8pi and volume at y axis is 128pi/5

btw this is exactly what the textbook says but without the sketch

 

[Peeik]

Re: 2012 HSC MX1 Marathon

OMG.....carrot, u have to see the question in the tbook....it changes everything. Timske...look at my answer, i think it's correct now. The fact that the question said "hint: subtraction of volumes will be neccesary" means my answer looks right.

@Carrot: The textbook wants the volume of x=+root(y) only.

Carrot check my working out please, im off to bed

 

[Carrotsticks]

Re: 2012 HSC MX1 Marathon

its both x and y axis
volume at x axis is 8pi and volume at y axis is 128pi/5

btw this is exactly what the textbook says but without the sketch

Like peeik said, the picture changes everything.

 

[Examine]

Re: 2012 HSC MX1 Marathon

Need help again guys..

sec^2=cosec/cosec-sin

At least I'm getting better , went through 9 questions before I stumbled upon this.

I keep on ending with cos-cot whenever I try this question, it's kinda bugging me.

 

[SpiralFlex]

Re: 2012 HSC MX1 Marathon

Need help again guys..

sec^2=cosec/cosec-sin

At least I'm getting better , went through 9 questions before I stumbled upon this.

I keep on ending with cos-cot whenever I try this question, it's kinda bugging me.

Quack quack.



I will now ridicule a BOS member.

"By doing nothing, you are doing something". - A vegetable and a stick.



Using the six star formula,

(I am out of stars for the week, my shopping is done one a month.)

 

[Examine]

Re: 2012 HSC MX1 Marathon

Quack quack.



I will now ridicule a BOS member.

"By doing nothing, you are doing something". - A vegetable and a stick.



Using the six star formula,

(I am out of stars for the week, my shopping is done one a month.)

How did I not see that idek

 

[Examine]

Re: 2012 HSC MX1 Marathon

Oh I see what I did wrong now. (not gonna state it because I'd look like more of a dumbass the I do atm)

 

[SpiralFlex]

Re: 2012 HSC MX1 Marathon

Part of human condition is to make mistakes. You often see people at the top of their professions make the most ridiculous mistakes.

- Care needs to be taken young grasshopper. I once tried to convince a teacher 1/3 is greater than a half. I was obviously on sugar.

*GROWL*