The Student Room Group

Tangents and perpendiculars to the initial line (polar coordinates - fp2 new)

Just curious how to start this:

r = a ( 3 + root 5 cos pheta )

Find the polar coordinates of the points P & Q where the tangents to C are parallel to the initial line.

Obviously I need a y = r sin pheta approach but the equation I'm working with seems void of any sin's.

What shall I do?
Reply 1
lolwut?

you must multiply r by sin theta and then differentiate and set to zero.
Reply 2
Tyrotoxism
lolwut?

you must multiply r by sin theta and then differentiate and set to zero.


Ta.

P.S - It's probably a stupid question but no where in my textbook was that conveyed to me :/
Reply 3
Bugzy
Ta.

P.S - It's probably a stupid question but no where in my textbook was that conveyed to me :/


don't worry about it.
Reply 4
Original post by Tyrotoxism
lolwut?

you must multiply r by sin theta and then differentiate and set to zero.


What about r^2=a^2cos(2θ) ?

Came across this while solving mock papers and I got stuck. If I try the d(rsin(θ))/dθ method I get some weird equation with mixed powers of cosθ and sinθ.