Gradient of norm question

bluefrog

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Mar 31, 2020
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I have attempted the following question.
Find the coordinates of the point where the tangent to the curve y=x2+1y=x^2+1 at the point (2,5)(2,5) meets the normal to the same curve at the point (1,2)(1,2)

My attempt is as follows:
dydx(x2+1)=2xgradient: 2(2)=4 Tangent: y=4x+c5=4(2)+cy=4x3Norm: y=14x+c2=14(1)+c2+1/4=cy=14x+94Intersect: 4x3=14x+944(4x3)=x+916x12=x+917x21=0x=2117y=4(2117)3=3317(x,y)=(2117,3317)\frac{dy}{dx} (x^2+1)= 2x \\ \text{gradient: } 2(2) = 4 \\ \text{ Tangent: } y= 4x+c \Rightarrow 5 = 4(2) + c \Rightarrow y = 4x - 3 \\ \text {Norm: } y=-\frac{1}{4}x+c \Rightarrow 2=-\frac{1}{4}(1) +c \Rightarrow 2+1/4=c \Rightarrow y=-\frac{1}{4}x+\frac{9}{4} \\ \\ \text{Intersect: } \\ 4x-3=-\frac{1}{4}x+\frac{9}{4} \\ 4(4x-3)=-x+9 \\ 16x-12=-x+9 \\ 17x-21=0 \\ x=\frac{21}{17} \Rightarrow y = 4(\frac{21}{17}) -3=\frac{33}{17} \\ \Rightarrow (x,y)=(\frac{21}{17}, \frac{33}{17})
But the answer in the book is shown as, answer is at 3

1678551491715.png

No working out is shown, only the final answer is shown.
Can anybody suggest where I have gone wrong ?
 
I think I understand the question a bit better.

I got confused with what tangent the norm should be ofo_O. So clearly there are two tangents, the other being y=2xy=2x,
therefore the norm is y=12x+cy=-\frac{1}{2}x+c,
hence 2=12x+cc=212y=12x+2122=-\frac{1}{2}x+c \Rightarrow c=2\frac{1}{2} \Rightarrow y=-\frac{1}{2}x+2\frac{1}{2}

The intersect is therefore 4x3=12x+2124x-3=-\frac{1}{2}x+2\frac{1}{2}, which when continued resolves to the same in the book, as shown above.
 
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