Multiply any numbers two numbers apart mentally :: CuriousMath.com :: Math is an attitude: "Multiply any numbers two numbers apart mentally
Posted by: erlkonig on Mar 15, 2005 - 08:58 AM
Tricks, Rules & Methods This trick is quite simple. All you have to know is to square a number (multiple articles on that here at CuriousMath.com) and basic subtraction.
If the numbers are two numbers apart, you square the number in the middle (e.g. for 19*21, you square 20). Afterwards, you simply need to subtract 1.
For number 3 apart, you take the product of the middle numbers, and subtract 2.
With numbers more than 3 apart, you need to take the outer 2 numbers, and subtract the number of the two numbers being multiplied (higher number-lower number). Take the sum of the outer numbers, and from that the number you got as the end product.
If you find that confusing, here are a couple of examples: