Multiplying Consecutive Numbers

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

then n x (n+1) = n² + n

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

then (n-1) x n = n² – n

We can apply this for speed calculation,


for example 65 x 66

Use the short cut method for squaring numbers ending in 5 and square 65

then 65² = 4225 ( put 25 in the last places and multiply 6 and 7 and put in the front places )

now 65 x 66 = 4225 + 65 = 4290

Another example Suppose we have to multiply 78 x 79

Use the short cut method for squaring numbers ending in 9 and find the square of 79

79²= (79 + 1) ² – (79 + 80 ) = 6400- 159 = 6241

so  applying the method 78 x 79 = 6241 – 79 = 6162