Gravitational Potential Questions

A particle of mass $M$ is situated at the centre of a spherical shell of same mass and radius $a$. The gravitational potential at a point situated at $\frac{a}{2}$ distance from the centre, will be:

Infinite number of bodies, each of mass $2\text{kg}$ are situated on the x-axis at distances $1\text{m},2\text{m},4\text{m},8\text{m},\dots$, respectively, from the origin. The resulting gravitational potential due to this system at the origin will be: