If 30 times 30 equals 900 why does 29 times 31 equal 899?

Answer 1

(x-1)*(x+1) = x2 - x + x - 1 = x2 - 1

So, when x = 30,

29*31 = 30*30 - 1

Or, since 30*30 = 900, 29*31 = 900-1 = 899