Reasoning · Grade 2-1 Large and Small Shapes
Problem
The side count equals how many outer corners the boundary turns at.
A shape's name comes from how many straight sides its outline has, so I only need to watch the outside edge, not the inside cut lines.
2.G.A.1Draw A DiagramLine the outer edges up straight and only three corners remain: a triangle.
If the added pieces only extend two existing straight edges and a flat base, no new corner appears, so the outline stays a triangle.
2.G.A.1Draw A DiagramIn a row or an L, as in the example, four corners give a quadrilateral.
Lining up 3 pieces so the boundary turns exactly four times reproduces the example's four-sided result.
2.G.A.1Draw A DiagramA piece jutting out adds one corner, making a pentagon.
Each extra outside corner adds one side, so going from 4 corners to 5 corners turns a quadrilateral into a pentagon.
2.G.A.1Make A Systematic ListEvery extra corner on the outside edge adds exactly one more side, so five corners make a pentagon.
Walking the outline, one straight side runs between each corner and the next, so corners and sides come in matching pairs.
Only the outside boundary decides the shape's name, and that boundary is the straight pieces of it laid end to end.
Zig-zag them so no edges line up and you get a hexagon.
The most jagged way to join 3 pieces gives the most outside corners, which is how 3 small pieces can still make a 6-sided shape.
2.G.A.1Make A Systematic ListWhen you glue small pieces together, only the OUTSIDE corners matter - count them to know if you made a triangle, quadrilateral, pentagon, or hexagon!