Hi,

     is it nesseccary that when a solid is cut(cross-section),it should form the same shape.if it is,then why is it different in the cross section      of a cylinder?!!!when it is cut vertically it's plane face obtained is a rectangle,and when it is cut horizontally it's plane face obtained is a circle.

Dear Student!

It is not necessary that when a solid is cut, it should form the same shape.

When the cylinder is cut along the plane making an angle α, then the cross-section of the plane face obtained is an oval shape.

When it is cut vertically, the plane face obtained is a rectangle and when it is cut horizontally, the plane face obtained is a circle.

Similarly, when  a cone is cut horizontally, the plane face obtained is a circle. When a cone is cut vertically, the plane face obtained is a triangle. 

Cheers!