Task from textbook (p98 Brainworks No. 12)
Suppose the 9 dates in a certain month are enclosed by a rectangle as shown in the diagram
(a) Explain a quick way to calculate the sum of the 9 numbers.
(b) Let n be the number at the top left hand corner of the rectangle. Express the sum of the 9 numbers in terms of n.
(c) Let m bet he middle number in the rectangle. Express the sum of the 9 numbers in terms of m.
(d) Write down an equation relating m and n.
(e) Describe some other interesting properties about the numbers within the rectangle.