posted by Jen on .
Find normals to the curve xy+2x-y=0 that are parallel to the line 2x+y=0
I have the answer: at(-1,-1), y=-2x-3, and at (3,-3), y=-2x+3
How do we get this? Thanks.
xy + 2x - y=0
x dy/dx + y + 2 - dy/dx=0
dy/dx (x-1)= -y-2
dy/dx= - (y+2)/(x-1)
The normal to this has a slope of
but this has to be parallel to a slope of -2
(x-1)/(y+2) = -2
x-1= -2y -4
x= -(2y +3 ) and the x,y has to be on the
curve xy + 2x - y=0 ,or
(-(2y +3 )y) + 2(-(2y +3 )) - y=0
2y^2 + 8y + 6=0
y^2 + 4y +3=0
and you have your two y solutions, put these back into the original equation to get the x values.
Sorry about the transcription error in the first post. Thanks for catching it.