precalculus
posted by Tati .
Convert from polar to rectangular r=3cos2theta

x= r cosTheta=3cos(2Theta)cosTheta
y= r sin Theta=3cos(2Theta)sinTheta 
In google type :
r=3cos2theta
When you see list of results click on :
Advanced Math: precal, trig identities, conversion
You will see answer. 
Identities r everywhere!!! :/, thank you 4 the info appreciated