c1 - Differentiation help Watch

longsightdon
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#1
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1. find the equation of the tangent to the curve with equation
y= x^3-3x^2-2x-6
at the point where it crosses the y-axis

2. Given the function f(x)=x^3+5x^2-x-4 find
a) f' (-2) b) the values of a such that f'(a) =56

3. find the equation of the normal to y=x^4-4x^3 at the point
for which x= 0.5


I've done the whole of the exercise except for these three. Any help guys?
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TenOfThem
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(Original post by longsightdon)
1. find the equation of the tangent to the curve with equation
y= x^3-3x^2-2x-6
at the point where it crosses the y-axis
It is hard to see why this one is causing a problem - what have you tried
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TenOfThem
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(Original post by longsightdon)
2. Given the function f(x)=x^3+5x^2-x-4 find
a) f' (-2) b) the values of a such that f'(a) =56
Have you found f ' (x)
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TenOfThem
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(Original post by longsightdon)

3. find the equation of the normal to y=x^4-4x^3 at the point
for which x= 0.5
Again - this seems very straightforward - what have you done so far
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longsightdon
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(Original post by TenOfThem)
It is hard to see why this one is causing a problem - what have you tried
i get this one now
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longsightdon
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(Original post by TenOfThem)
Have you found f ' (x)
no idea how to do that
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longsightdon
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(Original post by TenOfThem)
Again - this seems very straightforward - what have you done so far
nothing, i dont know how to do it
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TenOfThem
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(Original post by longsightdon)
i get this one now

(Original post by longsightdon)
no idea how to do that

I a struggling to see how you did (a) if you do not know how to differentiate
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longsightdon
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(Original post by TenOfThem)
I a struggling to see how you did (a) if you do not know how to differentiate
I wouldn't ask for help if I didn't know what to do... Please can you help?...
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TLHroolz
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(Original post by longsightdon)
1. find the equation of the tangent to the curve with equation
y= x^3-3x^2-2x-6
at the point where it crosses the y-axis

2. Given the function f(x)=x^3+5x^2-x-4 find
a) f' (-2) b) the values of a such that f'(a) =56

3. find the equation of the normal to y=x^4-4x^3 at the point
for which x= 0.5


I've done the whole of the exercise except for these three. Any help guys?
F'(x) is the same as dy/dx
So, for questions 2, differentiate.
Then for part a) plug in x=-2 into the differentiated function

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longsightdon
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(Original post by TLHroolz)
F'(x) is the same as dy/dx
So, for questions 2, differentiate.
Then for part a) plug in x=-2 into the differentiated function

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ok thank you. What do i do for b)
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Thinking_Aloud
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(Original post by longsightdon)
ok thank you. What do i do for b)
For B, you know what dy/dx is from part a so you put that equal to 56 and rearrange the equation to find x (or 'a' as it says in the question).
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TLHroolz
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(Original post by longsightdon)
ok thank you. What do i do for b)
So, they've replaced f'(x) with f'(a) and told you that it equals 56, so put x=a and set the whole equation equal to 56 (probably a quadratic)

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longsightdon
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(Original post by Thinking_Aloud)
For B, you know what dy/dx is from part a so you put that equal to 56 and rearrange the equation to find x (or 'a' as it says in the question).
(Original post by TLHroolz)
So, they've replaced f'(x) with f'(a) and told you that it equals 56, so put x=a and set the whole equation equal to 56 (probably a quadratic)

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ok thanks guys
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longsightdon
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(Original post by TLHroolz)
So, they've replaced f'(x) with f'(a) and told you that it equals 56, so put x=a and set the whole equation equal to 56 (probably a quadratic)

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dya think you could help me with no 3?
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TLHroolz
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(Original post by longsightdon)
dya think you could help me with no 3?
So, again, differentiate. Do you know how to work out the gradient of normals from the coordinate geometry chapter?

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longsightdon
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(Original post by TLHroolz)
So, again, differentiate. Do you know how to work out the gradient of normals from the coordinate geometry chapter?

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im getting really confused w/ it
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davros
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(Original post by longsightdon)
dya think you could help me with no 3?
For Q3, do you know how the normal relates to the tangent? In particular, how do you get the gradient of the normal if you know the gradient of the tangent at a point?
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longsightdon
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(Original post by davros)
For Q3, do you know how the normal relates to the tangent? In particular, how do you get the gradient of the normal if you know the gradient of the tangent at a point?
is it perpendicular?
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davros
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(Original post by longsightdon)
is it perpendicular?
Correct!

And what do you know about the gradients of perpendicular lines?
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