Question #226678

The normal to the parabola y2= 4ax at the point P(at2 , 2at) meets the x-axis at A. Find the equation of the locus of the midpoint of AP as t varies.


1
Expert's answer
2021-08-26T14:31:53-0400

Solution:

To find the normal;

At A(y,x) but y=0

Since;

y2=4axy^2=4ax

2ydydx=4a\Rightarrow 2y\frac{dy}{dx}=4a

dydx=2ay\Rightarrow \frac{dy}{dx}=\frac{2a}y

Slope of the tangent at

P(at2,2at)P(at^2,2at) ;m1=dydx=2a2atm_1=\frac{dy}{dx}=\frac{2a}{2at} =1t\frac1t

Hence slope of the normal at P is given as;

m2=11tm_2=\dfrac{-1}{\dfrac1t} =t-t

Therefore equation of the normal is given by;

y=m2x+cy=tx+cy=m_2x+c \\\Rightarrow y=-tx+c

Put x=at2,y=2atx=at^2,y=2at

So, c=2at+at3c=2at+at^3

Thus, tx+y=2at+at3tx+y=2at+at^3

Hence;

y2at=t(xat2)y-2at=-t(x-at^2)

But y=0

2at=tx+at3-2at=-tx+at^3

Rewrite as;

tx=at3+2attx=at^3+2at

Divide by t,we get;

x=at2+2ax=at^2+2a

Take the midpoint of AP to be B(h,k)

Here;

h=at2+2a+at22h=\frac{at^2+2a+at^2}{2} =at2+aat^2+a

k=0+2at2k=\frac{0+2at}{2} =atat

Since ;

y2=4axy^2=4ax

We have the equation of the locus as t varies given by at P;

(2at)2=4a(at2)(2at)^2=4a(at^2)

Using h and k ,we can make the following substitutions;

(2×k)2=4a(ha)(2×k)^2=4a(h-a)

4k2=4ah4a2\Rightarrow 4k^2=4ah-4a^2

Rewrite as ;

k2=aha2k^2=ah-a^2

k2ah+a2=0\Rightarrow k^2-ah+a^2=0

Writing it in terms of x and y as;

y2ax+a2=0y^2-ax+a^2=0 which is the standard locus at midpoint AP.


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