median
don steward
mathematics teaching 10 ~ 16

Tuesday, 4 December 2007

that's magic that is

write 8547 on the board





ask students to multiply this number by any digit
then they multiply this new answer by 13 (without a calculator)

what happens?
for any digit?

what is special about 8547 I wonder...
let's try to factorise it (using a calculator maybe)
[it is 3 x 7 x 11 x 37 or (7 x 11) x (3 x 37)]

what happens when you multiply 7 x 11 by 13?
and what is 3 x 37?
what happens when you multiply these two bits together?

a similar task might be to multiply 910091 by any digit and then by 11

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