# Math Help - Equation for a surface

1. ## Equation for a surface

Let L be the line y=7, x=5z. if we rotate L around the x-axis we get a surface whose equation is Ax^2+By^2+Cz^2=1
Find A, B, C

I got B=1/49 by setting x and z=0. Any hints on how to get A and C? I can show A=-25C by plugging in point (1,7,5) thinking I could get 2 equations with 2 unknowns thinking I could just use rref but all the second equation I've been able to get just says A=C=0 which is incorrect.

2. The point $(0,0,7)$ also lies on the surface giving $C=1/49$. You can now substitute this in to get $A$.

3. Originally Posted by aka
Let L be the line y=7, x=5z. if we rotate L around the x-axis we get a surface whose equation is Ax^2+By^2+Cz^2=1
Find A, B, C

I got B=1/49 by setting x and z=0. Any hints on how to get A and C? I can show A=-25C by plugging in point (1,7,5)
No, (1, 7, 5) is not on the surface. Since "x= 5z" you want (5, 7, 1).

thinking I could get 2 equations with 2 unknowns thinking I could just use rref but all the second equation I've been able to get just says A=C=0 which is incorrect.