> simplify(pi*r^2 * (r^2 + (8100/(pi*r^4))));

> diff(1/r^2*(r^6*pi+8100),r);

> solve(-2/r^3*(r^6*pi+8100)+6*r^3*pi,r);

4050^(1/6)/pi^(1/6),

(1/2+1/2*I*3^(1/2))*4050^(1/6)/pi^(1/6),

(-1/2+1/2*I*3^(1/2))*4050^(1/6)/pi^(1/6),

-4050^(1/6)/pi^(1/6),

(-1/2-1/2*I*3^(1/2))*4050^(1/6)/pi^(1/6),

(1/2-1/2*I*3^(1/2))*4050^(1/6)/pi^(1/6)

LOL I got too many answers, no wonder why it says I'm a newbie.