Question 759678
{{{x^2+y^2=400}}} <--> {{{x^2+y^2=20^2}}}
is the equation of a circle with radius {{{20}}}, centered at the origin, O(0,0).
{{{drawing(300,300,-25,25,-25,25,
grid(0),circle(0,0,20),
blue(line(-25,12,25,12)),
blue(line(16,-25,16,25)),
green(line(-16,12,16,-12)),
locate(17,15,P(x,y)),
locate(16.3,-12,Q(x,-y)),locate(-25,15,R(-x,y)),
line(0,0,16,12),locate(3,2.5,theta)
)}}} {{{x=20cos(theta)}}} {{{y=20sin(theta)}}}
The area of right triangle PQR can be calculated as
{{{(PQ)(PR)/2=(2y)(2x)/2=2xy=2*(20cos(theta))(20sin(theta))=80cos(theta)sin(theta)}}}