21 Mar '06 10:03

I was wondering if anyone might contribute some interesting ways to perform mental arithmacy quickly. For instance, for squaring numbers ending in five, you take the numbers infront of the five and multiply it by that plus one more, and then put 25 at the end. Proof:

Let n denote the numbers after the 5,

(10n+5)^2 = 100n^2 + 100n + 25

= 100n (n+1) + 25

I find this to be a good game when in the cars. When I see a licence plate I square the number. But I find it difficult to square four digit numbers without developing a head ache. Any suggestions?

Let n denote the numbers after the 5,

(10n+5)^2 = 100n^2 + 100n + 25

= 100n (n+1) + 25

I find this to be a good game when in the cars. When I see a licence plate I square the number. But I find it difficult to square four digit numbers without developing a head ache. Any suggestions?