What does complement means in vedic maths?

Answer 1

The term complement is not restricted to Vedic mathematics - it is a fairly common term. The complement of a number is another number in which each digit, when added to the corresponding digit in the original number, will give you a value exactly one less than the base in which the number is expressed.

Most people tend to work in decimal though, and if you are strictly using decimal notation (as opposed to binary for example), then you can simplify that definition to say:

The complement of a number is another number that, when added to the original, will have the same number of digits, and each one of them will be a "9".

For example, the decimal complement of 12345 is 87654. If you add the two numbers together, you'll get 99999. Similarily, the decimal complement of 8675309 is 1324690.

Note that the complementary value of a number will be entirely dependent on the base in which it's expressed. For example, in hexadecimal (base 16), the complement of the number 6 would be 9. In decimal on the other hand, it would be 3.