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C1 1 mark question! :(

OCR Jun 10, Q8iii)

Here's the full question

i) express 2x^2 + 5x in the form 2(x+p)^2 + q

Answer: 2(x+ 5/4)^2 - 25/8

ii) State coords of the minimum points

(-5/4,-25/8)

PART THREE) State the equation of the normal to the curve at its minimum point..

So we know x coord of the minimum point is -5/4

How do work out the equation of normal? ( I know it should work out to be -1) so i assume the answer is 4/5? I dunno what am doing :P

..

Can you kindly guide me and if you do know the answer, can you tell me how you worked it Out :smile:

Thanks

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Reply 1
equation of normal
is the perpendicular gradient of your tangent

and then you just slot the numbers in the formula y-y1-m(x-x1) where m is the gradient
Reply 2
Original post by dongonaeatu
equation of normal
is the perpendicular gradient of your tangent

and then you just slot the numbers in the formula y-y1-m(x-x1) where m is the gradient


All that for one mark? I doubt that lol!

Must be something else
Reply 3
Original post by jameslad
OCR Jun 10, Q8iii)

Here's the full question

i) express 2x^2 + 5x in the form 2(x+p)^2 + q

Answer: 2(x+ 5/4)^2 - 25/8

ii) State coords of the minimum points

(-5/4,-25/8)

PART THREE) State the equation of the normal to the curve at its minimum point..

So we know x coord of the minimum point is -5/4

How do work out the equation of normal? ( I know it should work out to be -1) so i assume the answer is 4/5? I dunno what am doing :P

..

Can you kindly guide me and if you do know the answer, can you tell me how you worked it Out :smile:

Thanks


so you want to find out a gradient and you have
(-5/4,-25/8)
so y2-y1/x2-x1 gives you the gradient of the tangent

you want the gradient of the normal which is perpendicular

can you do this
Reply 4
Original post by jameslad
All that for one mark? I doubt that lol!

Must be something else


well the gradient of the normal is perpendicular to the gradient of the tangent

so if the gradient of a tangent is 2, gradient of the normal is -1/2
Reply 5
Original post by jameslad
OCR Jun 10, Q8iii)

Here's the full question

i) express 2x^2 + 5x in the form 2(x+p)^2 + q

Answer: 2(x+ 5/4)^2 - 25/8

ii) State coords of the minimum points

(-5/4,-25/8)

PART THREE) State the equation of the normal to the curve at its minimum point..

So we know x coord of the minimum point is -5/4

How do work out the equation of normal? ( I know it should work out to be -1) so i assume the answer is 4/5? I dunno what am doing :P

..

Can you kindly guide me and if you do know the answer, can you tell me how you worked it Out :smile:

Thanks


x=54 x = - \dfrac54 is the minimum point, see the graph, from the graph we can see that the equation of the normal will be x=54 x= -\frac54
(edited 11 years ago)
isnt it just y=-25/8
The gradient at the minimum point is 0, is it not.

Use that OP.
Original post by raheem94
x=54 x = - \dfrac54 is the minimum point, see the graph, from the graph we can see that the equation of the normal will be x=54 x= -\frac54


wont that be the normal?
oh sorry just notice the question asked for the normal
(edited 11 years ago)
Original post by jameslad
OCR Jun 10, Q8iii)

Here's the full question

i) express 2x^2 + 5x in the form 2(x+p)^2 + q

Answer: 2(x+ 5/4)^2 - 25/8

ii) State coords of the minimum points

(-5/4,-25/8)

PART THREE) State the equation of the normal to the curve at its minimum point..

So we know x coord of the minimum point is -5/4

How do work out the equation of normal? ( I know it should work out to be -1) so i assume the answer is 4/5? I dunno what am doing :P

..

Can you kindly guide me and if you do know the answer, can you tell me how you worked it Out :smile:

Thanks


Well the normal is the perpendicular to the tangent.

If it's a minimum point - what is the gradient of the tangent ALWAYS?
Reply 10
Original post by raheem94
x=54 x = - \dfrac54 is the minimum point, see the graph, from the graph we can see that the equation of the tangent will be x=54 x= -\frac54


Equation of tangent? It's asking for normal..
isn't it just x=-5/4?
Reply 12
zero?
Original post by jameslad
Equation of tangent? It's asking for normal..


He meant normal.
Reply 14
Original post by jameslad
Equation of tangent? It's asking for normal..


By mistake wrote tangent, i have corrected it.

Sorry for the confusion it caused.
Reply 15
That is the answer, but i'm just wondering .. why
Original post by hassi94
Well the normal is the perpendicular to the tangent.

If it's a minimum point - what is the gradient of the tangent ALWAYS?


minimum point- the gradient is positive is this right
Original post by jameslad
zero?


Yes that's right so the tangent is just the horizontal line going through that point. Therefore the normal is the vertical line going through that point i.e. x = -5/4
Reply 18
Original post by dongonaeatu
so you want to find out a gradient and you have
(-5/4,-25/8)
so y2-y1/x2-x1 gives you the gradient of the tangent

you want the gradient of the normal which is perpendicular

can you do this


The gradient of the tangent is zero.

So for finding the gradient of the normal will we do 10 \displaystyle - \frac10

We can't do this here!
Original post by dongonaeatu
minimum point- the gradient is positive is this right


No. How do we find minimum points using calculus?

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