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Triangular numbers can be represented by a triangular array of squares.
Imagine two copies of a triangular array of squares. Can you picture how to fit them together to make a rectangle? What is special about the dimensions of your rectangle?
Use the activity below to test your ideas.
What do you notice about doubling triangular numbers?
Experiment with different triangle numbers and explain what is special about the rectangles made from two identical triangle numbers.
Can you write down the dimensions of the rectangle made from two copies of the 250th triangle number? Can you use this to work out the 250th triangle number?
Check - your answer should be somewhere in this list:
29184, 31375, 586594, 908475, 2092035
Deduce a strategy for working out any triangle number.
Consider the following numbers: $4851, 6214, 3655, 7626, 8656$.
Which are triangle numbers?
How did you decide?
Do any triangle numbers end $000$?
You may also be interested in reading the article Clever Carl, the story of a young mathematician who came up with an efficient method for adding lots of consecutive numbers.
Imagine you are suspending a cube from one vertex and allowing it to hang freely. What shape does the surface of the water make around the cube?
Given the nets of 4 cubes with the faces coloured in 4 colours, build a tower so that on each vertical wall no colour is repeated, that is all 4 colours appear.
Draw a square. A second square of the same size slides around the first always maintaining contact and keeping the same orientation. How far does the dot travel?