Year 12s: what will be the first way you research your future options? >>

Edexcel C3,C4 June 2013 Thread

Scroll to see replies

Can you write the Q.

At time t seconds, the surface area of a cube is A cm2 and the volume is V cm3, The surface area of the cube is expanding at a constant rate 2cm2s-1

Show that dv/dt = 1/2V^1/3

Reply 761

masryboy94

Original post by 6vor6kas6

Urm, guys, I found one my major weaknesses in C3. Trigonometric Transformations. I always seem to get a lot of exercises which require to transform a trigonometric equation into something else wrong. Any tips?

what exactly is it your struggling with in trigonometric transformations? is it noticing which identity to use?

Reply 762

justinawe

[INDENT]

Original post by otrivine

so am I doing it wrong, thought that I was doing correct so far

You've done it right so far. They've given you dA/dt as 2, so you can sub that in.

To find dA/dV, you need to get A in terms of V, which was what I was trying to help you do in my previous post.

Reply 763

otrivine

Original post by justinawe

[INDENT]

You've done it right so far. They've given you dA/dt as 2, so you can sub that in.

To find dA/dV, you need to get A in terms of V, which was what I was trying to help you do in my previous post.

how do u do

Reply 764

Kvothe the Arcane

Original post by otrivine

At time t seconds, the surface area of a cube is A cm2 and the volume is V cm3, The surface area of the cube is expanding at a constant rate 2cm2s-1

Show that dv/dt = 1/2V^1/3

Well

V=h^3 \ and \ A \ = \ 6h^2

\dfrac{dA}{dt}=2

A=6(h^3)^\frac{2}{3}

A=V^\frac{2}{3}

Find

\dfrac{dA}{dV}

(edited 11 years ago)

Reply 765

otrivine

Original post by reubenkinara

Well

V=h^3 \ and \ A \ = \ 6h^2

\dfrac{dA}{dt}=2

Make h the subject in one of the equations, So either

h=V^\dfrac{1}{3}

or the other then substitute so in the case I gave

A=6(h^\frac{1}{3})^6

A=V^6

Find

\dfrac{dA}{dV}

According to the man on exam solution, he said its not a good idea to make subject its better to split the differentiation

Reply 766

justinawe

Original post by reubenkinara

Well

V=h^3 \ and \ A \ = \ 6h^2

\dfrac{dA}{dt}=2

Make h the subject in one of the equations, So either

h=V^\dfrac{1}{3}

or the other then substitute so in the case I gave

A=6(h^\frac{1}{3})^6

A=V^6

Find

\dfrac{dA}{dV}

V=h^{3}

, not

h^{\frac{1}{3}}

A = 6(h^3)^{\frac{2}{3}} = 6V^{\frac{2}{3}}

Reply 767

justinawe

Original post by otrivine

According to the man on exam solution, he said its not a good idea to make subject its better to split the differentiation

yes... but you need to find dA/dV after you do that!

Reply 768

otrivine

Original post by justinawe

yes... but you need to find dA/dV after you do that!

and how would u find DA/DV ? do u need to relate the area and volume into one equation and differentiate

Reply 769

justinawe

Original post by otrivine

and how would u find DA/DV ? do u need to relate the area and volume into one equation and differentiate

Yes... as reubenkinara and myself have been saying, find A in terms of V, and then differentiate.

Reply 770

otrivine

Original post by justinawe

Yes... as reubenkinara and myself have been saying, find A in terms of V, and then differentiate.

but anoter method apart from rearragning

can you express DA/DV in terms of da/dt x dt/dv

Reply 771

justinawe

Original post by otrivine

but anoter method apart from rearragning

can you express DA/DV in terms of da/dt x dt/dv

You could if you had dt/DV, but you don't

Reply 772

otrivine

Original post by justinawe

You could if you had dt/DV, but you don't

so can I not do that method of splitting do I have to rearrange?

Reply 773

usycool1

Lost 3 marks on a C4 paper because I thought 2 + (0 x 1) = 3 (it was a vector question) :facepalm2:

I make so many silly mistakes, grr :mad:

Reply 774

Kvothe the Arcane

Original post by justinawe

You could if you had dt/DV, but you don't

Strange, I'm getting

3V^\frac{1}{3}

as my answer.

Reply 775

Kvothe the Arcane

Original post by usycool1

Lost 3 marks on a C4 paper because I thought 2 + (0 x 1) = 3 (it was a vector question) :facepalm2:

I make so many silly mistakes, grr :mad:

I make them as well! I think we all do.

Reply 776

justinawe

Original post by otrivine

so can I not do that method of splitting do I have to rearrange?

You do that method of splitting for dV/dt.

You are given dA/dt, and you can quite easily find dA/dV.

EDIT: Actually, looking back at your previous posts, I think you split dV/dt wrong. Can you repost your working here.