Suppose
we have to multiply two consecutive numbers say n and n+1

then n x (n+1) = n

If we take it in the reverse way, say (n-1) and n

then (n-1) x n = n

We can apply this for speed calculation,

then n x (n+1) = n

^{2}+ nIf we take it in the reverse way, say (n-1) and n

then (n-1) x n = n

^{2}– nWe can apply this for speed calculation,