Questions about Mathematics

(1)
We can change polar coordinate into Cartesian coordinate.
For polar coordinate A(a,b), a denotes the distance of the point A from the origin O, b denotes the angle from the x-axis to OA.
Let A(x,y) can be transformed into the form of Cartesian coordinate.
x=a cos b
y=a sin b
Therefore A(a,b)=A(a cos b,a sin b) in the form of Cartesian coordinate.
Your example can be done by following method:
(2,240) in Cartesian coordinate form is (-1,-2 cos 60)
(11,150) in Cartesian coordinate form is (-11 cos 30,5.5)
Therefore the mid pt. (p,q) is:
p=(-1-11 cos 30)/2
q=(-2 cos 60-5.5)/2
(2)
I do not know how to solve cubic equation. I am sorry for that.

(1)可用中點公式,已知A(x1,y1),B(x2,y2),則中點為((x1+x2)/2,(y1+y2)/2)
所以你要求的中點是((2+11)/2,(240+150)/2)即(13/2,195)

(2)用電腦作圖求得
a約等於-1.23