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Homework Statement
Find the points at which the following polar curves have a horizontal and vertical tangent line.
(a) r = 3 + 6 cos(theta)
Homework Equations
The Attempt at a Solution
x = r cos(theta) = (3 + 6 cos(theta)))cos(theta) = 3cos(theta) + 6 cos(theta)^2
y = rsin(theta) = (3 + 6cos(theta) )sin(theta) = 3 sin(theta) + 6 cos(theta)sin(theta)
dy/dtheta = 3 cos(theta) + 6[cos(theta)^2 - sin(theta)^2]
dx/dtheta = -3 sin(theta) - 12 cos(theta)sin(theta)
dy/dx = - [cos(theta) + 2[cos(theta)^2 - sin(theta)^2]]/[sin(theta) - 4cos(theta)sin(theta)]
for vertical tangent
sin(theta) = 4 cos(theta)sin(theta)
1 = 4 cos(theta)
theta = arccos(1/4)
for horizontal tangent
cos(theta) + 2cos(theta)^2 = 2sin(theta)^2 = 2(1-cos(theta)^2) = 2 - 2cos(theta)^2
cos(theta) + 2cos(theta)^2 + 2cos(theta)^2 - 2 = 0
4cos(theta)^2 + cos(theta) - 2 = 0
cos(theta) = (-1 +/- sqrt(1 - 4(-2)(4) ))/(2(4)) = (-1 +/- sqrt(33) )/8
theta = arccos( (-1 +/- sqrt(33) )/8 )
I don't see what I'm doing wrong, the answer key says that the vertical tangent occurs at theta = arcsin(1/4) and the horizontal tangents occur at arcsin( (-1 +/- sqrt(33) )/8 )
Thanks for any help!