x=ρcos θ，y=ρsin θ

x^2+y^2=ρ^2

ρ^2=ρ*4cos θ=4x

So the general equation is x 2+y 2 = 4x.

Finishing (x-2) 2+y 2 = 4.

According to the definition domain, it is a semicircle above the horizontal axis.

(2)

Make a semicircle tangent through point p.

According to the geometric principle, the length of tangent = root number (4 2-2 2) = root number 12=2 times root number 3.

(According to Pythagorean Theorem)

Square of tangent =|PA|*|PB|= 12

(The formula comes from the cutting line theorem)