Edexcel Core 3 - 21st June 2016 AM

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I swear I'm having a meltdown. It's not even that C3 is hard. It's that no matter how much time I spend on a question, I always miss out on a random M1 mark. Do ED EXCEL really expect us to write identical working to them? Can I just skip a step since it'll probably be really obvious and implied or will they take away a mark even though I did the rest of the working out + got the correct answer

It's okay, it's just pre-exam nerves freaking you out. Been there.

Write down as much as you feel like. Don't go overboard with like dividing, factoring etc but don't cut out anything that isn't simple and then they have nothing to dock you marks for.

Reply 661

Ainsleyy

In the june 2015 c3 paper can anyone tell me how they do question 8 part b? Part a is pretty hard too but I can see what they did.

(edited 7 years ago)

Reply 662

Kevin De Bruyne

Original post by Mattematics

Guys, why can't you take natural logs of this equation to find t (part c)?

You would struggle to seperate the two exponential terms as it'd be something like ln(constant) = ln(18000e^-0.2t + 4000e^-0.1t) which is a nightmare.

Reply 663

Don Pedro K.

@Zacken or anyone else, for part d of this question:

d/dx[fg(x)] = 2x + 6xe^{x^2}.

why is it that when you factorise d/dx[fg(x)] to x(2 + 6e^{x^2}) and set it equal to x(xe^{x^2} + 2) in order to cancel the x on both sides, you only get x = 6 and miss the x = 0? I mean, I don't think there's anything mathematically incorrect by cancelling the x on both sides is there?

MS:

https://07a69ccf283966549a9350d1a66951a7bc96e2dc.googledrive.com/host/0B1ZiqBksUHNYZ0JQM1NRcmdHdXM/January%202009%20MS%20-%20C3%20Edexcel.pdf

Reply 664

Kevin De Bruyne

Original post by Ainsleyy

In the june 2015 c3 paper can anyone tell me how they do part b? Part a is pretty hard too but I can see what they did.

Use the result from part (a) and then get something in terms of sin and cos, I think?

Reply 665

Mattematics

Original post by SeanFM

You would struggle to seperate the two exponential terms as it'd be something like ln(constant) = ln(18000e^-0.2t + 4000e^-0.1t) which is a nightmare.

I'm being stupid - why can't you split the right hand side up into two parts?

Reply 666

math42

Original post by Mattematics

Guys, why can't you take natural logs of this equation to find t (part c)?

why would you?

Reply 667

Kevin De Bruyne

Original post by Mattematics

I'm being stupid - why can't you split the right hand side up into two parts?

No worries, everyone hits this brick wall at one point or another in Maths.

There is no such law that you can deal with that. The laws of logs are that ln(ab) = ln(a) + ln(b) and ln(a/b) = ln(a) - ln(b), and ln(a^b) = blna.

So.. nothing helps. Logs aren't linear - ln(a+b) is not the same as ln(a) + ln(b).

Reply 668

keres

Guys please help me with part b!
It's from an international paper for C34 and I have no idea how to get the values of k
x

Reply 669

Anon_98

Can someone help on this question pls:

I'm stuck on part b)

I basically equated the two ..equations and got x = 20 but the mark scheme also has x = -10/3. The working out they did was:

But I don't understand why.

Reply 670

math42

Original post by Don Pedro K.

@Zacken or anyone else, for part d of this question:

well you can't cancel the x if it's zero, that's dividing by 0

Reply 671

Don Pedro K.

Original post by Ainsleyy

In the june 2015 c3 paper can anyone tell me how they do question 8 part b? Part a is pretty hard too but I can see what they did.

Use what you proved in part a) and set it equal to 1/2. See if you can take it from there (remembering your standard trig identities).

Reply 672

Ainsleyy

Original post by SeanFM

Use the result from part (a) and then get something in terms of sin and cos, I think?

They dont even show the steps in the marks cheme they just jump to tan(theta) = -(1/3)...

I can do the first step and that was as far as I got:

(edited 7 years ago)

Reply 673

abro1089

Original post by Ainsleyy

In the june 2015 c3 paper can anyone tell me how they do question 8 part b? Part a is pretty hard too but I can see what they did.

Sub in the left hand side from the top prove that. Multiply across the bottom of the fraction and expand. Rearrange so you have sin/cos on one side and a fraction on the other side. That becomes tan, then solve

Reply 674

minjinoor

Original post by willmotg

If the powers are 2, the solutions are x=2 and x=-8. Is this the answer to the equation you've got?

Have you also been given a domain? E.g. must x>0

i've just relooked at it , you're completely right!

thanks

Reply 675

Pablo Picasso

Original post by keres

Guys please help me with part b!
It's from an international paper for C34 and I have no idea how to get the values of k
x

k > Turning points

Reply 676

Kevin De Bruyne

Original post by Ainsleyy

They dont even show the steps in the marks cheme they just jump to tan(theta) = -(1/3)...

I can do the first step and that was as far as I got:

after the first line of b (keeping in mind you want theta on its own somehow)

Multiply both sides by 2 and (costheta - sintheta).

Then take 1 * costheta away from both sides, take away 2 *sintheta away from both sides. (You could bring them to the same side but it'd be pointless).

So you have that 1costheta = -3sintheta.

Divide both sides by 3costheta to give you what they've got.