A-Level Edexcel Core Maths C3 June 2011 Q7b : ExamSolutions

[Parky]

You've made this look so much harder than it actually is. Using the quotient rule to differentiate means you can do this in about 3 minutes

[junaidsangal]

product rule seems to work the best imo
y=5[(2x+1)(x+3)]^-1
y=5t^-1
t=2x^2+7x+3

dy/dt = -5t^-2 .. 
dt/dx = 4x+7

dy/dx = -5(t)^-2 * 4x+7
simplifies to:

-5(4x+7) all over t^2

to sum up: dy/dx = -5(4x+7)÷(2x^2+7x+3)^2

[ExamSolutions_Maths]

It is in C4 and so you should at this level do it as [(2x+1)(x+3)]^-1 and use the chain rule.

[ExamSolutions_Maths]

It's not that long! You can use your method though, I agree.

[ExamSolutions_Maths]

@samjjak That's ok - pleased to hear that it helped.

[benqureshi]

If you split F(x) into partial fractions, you get f(x) = 2(2x+1)^-1 - (x+3)^-1
so you work out F'(x) = -4/(2x+1)^2 + 1/(x+3)^2
when you put in your x value, gradient of tangent pops out as -15/4 and its plain sailing from there.
just another method for anyone who finds this one a little taxing :)

[PimpmaPC]

Just use the quotient rule, it works. Implicit differentiation is in C4 so ur not supposed to know if for C3,

[zaklondon]

you differentiated it in a very long way, you could have multiplied out the bottom and raised the whole term to power -1 giving 5(2x^2 + 7x + 3)^-1 where you can easily use chain rule!

[ExamSolutions_Maths]

Why use the quotient rule when you have a constant on the top of the fraction.

[ExamSolutions_Maths]

It sounds like you are integrating rather than differentiating.

[Ace-qwgy]

Cant do this method as implicit differentiation not in core 3 spec but thanks anyway

[MrSandersHD]

Is it wrong to split the original fraction into partial fractions and then differentiate it creating natural logs?

[bobmarley]

Why can't you use the quotient rule????

[vHamadx]

Can I use the quotient rule? reply asap please!

[amirkh]

bruv everyones sleeping now, i havnt tried quotient rule but why don't u just use the simple chain rule ; u=2x^2+7x+3 and then u^-1 it ..., I'm awake till morning if u needed any help, good luck mate

[mrtictac]

What's wrong with the quotient rule?

[Alexbrainbox]

Ah, scratch that. Looks like this paper has had some positives mis-transcribed as negatives. ;_;

[mohammedaabid]

This video is going to confuse the hell out of you, use quotient rule instead

[amirkh]

na dude, can't sleep, I'm a night worker guy / lol

[vHamadx]

you should really go bed, its early in the morning dude!