Everybody is expected to recognise first 12 square numbers: 1,4,9,16...
but top flight students need to go a little further!
13X13= 169 (think 3X13=39)
14X14= 196 (think the same as above with the two last digits swapped)
15X15=225 (think 1.5 X1.5 =2.25, ie 1.5 + 0.75)
enjoy!
...and there are 5 more...
Comments