Mathematical trick: multiply a number by 9, 99 or 999

Wednesday, January 13, 2010

Very often I hear from students' math is one thing to genes and the affirmation that I hate more: "I am not inclined towards mathematics, but only for matters literary" I'm thinking that if a student says this is because believe someone has done this. In my opinion we are all potentially good at mathematics and literature.

Use the brain may be faster than using a calculator just to do some 'training and use some trick.

Here's the rule for multiplying a number by 9, 99 or 999

Multiplying a number by 9 means multiply by 10-1

Therefore, 9 × 9 and how to say 9x (10-1) from which 9 × 10-9 to 90-9 in 81 words

Here's another example: 78 × 9 = 780-78 = 702

To multiply a number by 99 to multiply the number 100-1

Therefore, 54 × 99 = 54x (100-1) = 5400-54 = 5346.

Multiply by 999 using the same method used for 9 and 99

42 × 999 = 42x (1000-1) = 42000-42 = 41958