Answer to Question #141007 in Analytic Geometry for Dhruv Rawat

Question #141007
Find the equation of the cone whose vertex is at the origin and base is the circle x=a, y^2+z^2=b^2.
1
Expert's answer
2020-10-29T19:49:06-0400

The sphere S intersects with the plane П \to x = a gives the circle

Cy2+z2=b2C \to y^2+z^2=b^2

Spp0=rS \to ||p-p_0||=r

with

r=bab2+a2r = \frac{b}{a}\sqrt{b^2+a^2}

p=(x,y,z),p0=(x0,y0,z0)p = (x,y,z), p_0 = (x_0,y_0,z_0)

x0=a+r2b2,y0=z0=0x_0 = a+\sqrt{r^2-b^2}, y_0=z_0=0

Consider the line Lp=λvL\to p=\lambda \vec{v} which passes by (0,0,0)

intersections S and L:

λvp0=r||\lambda\vec{v} - p_0||=r

λ2v22λv,p0+p02=r2\lambda^2||\vec{v}||^2-2\lambda\langle\vec{v}, p_0\rangle+||p_0||^2=r^2

λ=v,p0±v,p02v2(p02r2)\lambda=\langle\vec{v}, p_0\rangle \pm \sqrt{\langle\vec{v}, p_0\rangle^2-||\vec{v}||^2(||p_0||^2-r^2)}

the line L must be tangent to S hence

v,p02v2(p02r2)=0\langle\vec{v}, p_0\rangle^2-||\vec{v}||^2(||p_0||^2-r^2)=0

This is the locus defining the cone surface.

Putting v=(x,y,z)\vec{v}=(x,y,z) and applying the values for p0,rp_0,r we get

b4x2a4(y2+z2)2a2b2(y2+z2)+a2b2(x2+y2+z2)=0b^4x^2-a^4(y^2+z^2)-2a^2b^2(y^2+z^2)+a^2b^2(x^2+y^2+z^2)=0



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