Did you solve it? Try this triple Tetris teaser

Earlier today I set you three symmetry puzzles. Here they are again with solutions.

The most entertaining way to solve these problems is to cut the pieces out of paper and do the rearranging by hand. However, a generous reader made an interactive version available here.

1. Triangle twins

An easy one to start. These two ‘30-60-90’ triangles share a side length.

(Each triangle has internal angles of 30, 60, and 90 degrees: what you would get if you cut an equilateral triangle in half.)

How would you rearrange the two triangles without overlaps to get a shape with mirror symmetry, that is, one in which a line divides the shape into two halves, one half the reflection of the other.

Find BOTH solutions.

Solution

2. Tetromino triplets

This one for the tetris fans. Here are three L-shaped tetrominos (the technical term for a shape made from four squares joined along grid lines.)

Can you rearrange them with no overlaps so that the combined shape has a line of mirror symmetry?

There is one way to do it without flipping, and one way with flipping. (Imagine you had cut out the shapes. There is one solution just by sliding the shapes around, and one way in you have to pick one shape up and flip it around before putting it back on the table.)

Solution

3. Triomino quintuplets

Same again, this time with five L-triominos (i.e a shape made from three squares.) Can you rearrange them with no overlaps so that the combined shape has a line of mirror symmetry?

Find a solution where the line of symmetry is either parallel to, or perpendicular to, all the edges of all the triominoes. (So using the line of symmetry of an individual triomino does not count.)

Solution

Thanks to Donald Bell for today’s puzzles. Donald is a former director of the National Engineering Laboratory. If you would like to hear more about his passion for polyominoes, here’s a talk he gave about them.

I hope you enjoyed today’s puzzles. I’ll be back in two weeks.

I’ve been setting a puzzle here on alternate Mondays since 2015. I’m always on the look-out for great puzzles. If you would like to suggest one, email me.