a) charge placed at the centre of a shel is +q. Hence, a charge of magnitude -q will be induced to the Surface charge density at the inner surface of the shell is  given by the relàtion,

${\sigma }_1=\frac{\text{ Total charge }}{\text{ Inner surface area }}=\frac{-q}{4\pi r_1^2}$

A charge of +q is induced on the outer surface of the shel. A charge of magnitude Q is placed on the outer Surtace of the shell. Therefore, total charge on the outer surface of the shell is Q+ q. Surtace charge density at the outer

${\sigma }_2=\frac{\text{ Total charge }}{\text{ Outer surface area }}=\frac{Q+q}{4\pi r^2}$

(b) Yes

The electric field intensity inside a cavity is zero, even if the shell is not spherical and has any irregular shape.
lake a closed loop such that a part of it is inside the cavity along a field line while the rest is inside the
oNe One aes argeov sopi zero Decause tne neld
inside the conductor IS zero. Hence, electric field is zero, whatever is the shape.