$3.$ Given the electric flux density, $D=0\mathrm{.}5r^{2}drn\frac{C}{m^2}.$

$($a$)$ Find $E$ at point $P(r=4,=25,\frac{O}{?}=90).$

$($b$)$ Find the total charge within the sphere, given the circumference of a circle of the sphere,

$C=25\mathrm{.}133m$