Two-dimensional (2D) shapes are completely flat. 2D shapes can be found printed in a book or on a website. 2D shapes have sides and corners. A rectangle has 4 straight sides and 4 corners.

A mathematical problem solved by Susanna Heikkilä relates to the classification of quasiregularly elliptic 4-manifolds, ...

There are four identical triangular faces, so the total surface area is \(25 + 4 \times 15 = 85~cm^2\) ...

A mathematical problem solved by Susanna Heikkilä relates to the classification of quasiregularly elliptic 4-manifolds, ...