Difference of Squares
Two numbers the same distance from a friendly number (e.g. 98 × 102 from 100).
98 × 102 = 9996
98 and 102 sit 2 either side of 100, so this is (100 − 2)(100 + 2).
Multiplied out, the middle terms are +200 and −200 — they cancel exactly.
What survives is one square minus another: 10000 − 4 = 9996.
Your turn
Three to try in your head — tap what you get.
29 × 31
147 × 153
19 × 21
All 36 shortcuts: the Magic Tricks library.