Answer:
the shape we get is isosceles traingle.
Explanation:
given that a pyramid with square base. It means on the slant side this pyramid has triangles.
So, if we cut a cross section in this pyramid which is perpendicular to the base and passes through top vertex, we get an isosceles triangle whose base length is equal to side of square at base and other two equal side's length equals to the slant height of pyramid.
For better understanding figure is attached.