differentiation Watch

mrdetermination
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#1
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#1
hey guys i was wondering if some one could tell me where i am going wrong with this questions thanks

find the equation of the normal of the following curve at the given point



y=4 - 5x^-1/2 x=4

to get y i sub x into y (i dont usually wory about power so i ignore it at this stage is this correct to do this?)

i get -16

y=-16

okay now i look for the gradient called 'M'

i differicate

m= 5/2x^-3/2

again i ignore the power

M= 5/2 * 4
M= 10

OKAY NOW I HAVE
X=4 Y=-16 M=10
I CHANGE M TO -1/10

Y-Y1=M(X-X1) Y+16=-1/10(X-4) = 10Y+160=-X+4
= 10Y=-X-156



HOWEVER THIS IS NOT SEEN AS THE CORRECT ANSWER, CAN SOME TELL ME WHAT I DID WRONG
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TenOfThem
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#2
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(Original post by mrdetermination)
(i dont usually wory about power so i ignore it at this stage is this correct to do this?)
CAN SOME TELL ME WHAT I DID WRONG
here

when you ignored the power
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mrdetermination
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#3
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okay what do i do with the power how shall i proceed to it
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Barcelona'99
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#4
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(Original post by mrdetermination)
hey guys i was wondering if some one could tell me where i am going wrong with this questions thanks

find the equation of the normal of the following curve at the given point



y=4 - 5x^-1/2 x=4

to get y i sub x into y (i dont usually wory about power so i ignore it at this stage is this correct to do this?)

i get -16

y=-16

okay now i look for the gradient called 'M'

i differicate

m= 5/2x^-3/2

again i ignore the power

M= 5/2 * 4
M= 10

OKAY NOW I HAVE
X=4 Y=-16 M=10
I CHANGE M TO -1/10

Y-Y1=M(X-X1) Y+16=-1/10(X-4) = 10Y+160=-X+4
= 10Y=-X-156



HOWEVER THIS IS NOT SEEN AS THE CORRECT ANSWER, CAN SOME TELL ME WHAT I DID WRONG


I don't really understand this 'forget the power' idea. You cannot do that, the power is crucial when subbing values in. You have differentiated correctly. It is subbing x into y and x into dy/dx where you have gone wrong.
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TenOfThem
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(Original post by mrdetermination)
okay what do i do with the power how shall i proceed to it
Do you know what a power of -\frac{1}{2} is

Do you know what the value of 4^{-\frac{1}{2}} is
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mrdetermination
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#6
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(Original post by TenOfThem)
Do you know what a power of -\frac{1}{2} is

Do you know what the value of 4^{-\frac{1}{2}} is

yh thats 1/2
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TenOfThem
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#7
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(Original post by mrdetermination)
yh thats 1/2
So, what is the problem
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mrdetermination
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#8
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#8
(Original post by TenOfThem)
So, what is the problem

wow thank you really helped me out i completely understand now to reward you when i become king of this world i would spare you
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TenOfThem
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#9
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excellent

another one for the list
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