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'Multiply the Addition Square' printed from http://nrich.maths.org/
Take an addition table from $1$ to $10$ (or any other that you like
Now take a $3\times 3$ square of numbers on it, such as this
Now multiply the two diagonally
opposite corner numbers together:
And then find the difference between the two answers:
$5 \times 9 = 45$
$7 \times 7 = 49$
$49 - 45 = 4$
Now try the numbers in the green
What is the answer?
Investigate other $3\times 3$ squares.
What do you notice?
Can you explain it?
Now try with $2\times 2$ squares and $4\times 4$
Can you predict your answers? What is happening?