Free Math Tips

Mathematical trick: multiply by powers of 2

Wednesday, January 13, 2010

Students who follow courses in electronics and systems know that it is essential to know the binary system and as a result quickly calculate the powers of 2 of a number. As repeatedly explained below, the lecture method, with a little 'training will make you fast in mental calculation. To multiply a number by 2, 4, 8, 16, 32 or any other power of 2 is sufficient to double the product as many times...

Mathematical trick: multiply a number by 5, 25 or 125

Multiply by 5 Multiplying a number by 5 and multiply it by like 10 and then divide by 2. Remember that (obvious but better remembered) that multiplying a number by 10 means add one zero to the bottom of the number. 16 × 5 = (16 × 10) / 2 = 160 / 2 = 80 Another example: 82 × 5 = (82 × 10) / 2 = 820 / 2 = 410 Again 6840 × 5 = 68400 / 2 = 34200 Multiply by 25 Multiplying a number by 25 you multiply...

Mathematical trick: multiply by 11

To multiply a number by 11 and add enough pairs of numbers from right within the given number except the numbers on the edges that should be repeated. For example: consider the following product: 324 x 11 * From the right type 4 (the last number to the right of 324) first digit of the product; * Try the sum 4 +2 = 6 (the sum of 1 'and 2' figure), you get the second digit of the product; *...

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

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...

Dividing by 14

Tuesday, January 12, 2010

Sara Heikali builds on her divisibility test for seven: How can you know if a number with three or more digits is divisible by the number fourteen? Check if the last digit of the original number is odd or even. If the number is odd, then the number is not divisible by fourteen. If the number is even, then apply the divisibility rule for seven. (Keep in mind, the odd and even test is to see...

Dividing by 13

Here's a straightforward method supplied by Scott Fellows: Delete the last digit from the given number. Then subtract nine times the deleted digit from the remaining number. If what is left is divisible by 13, then so is the original number. Rafael Ando contributes: Instead of deleting the last digit and subtracting it ninefold from the remaining number (which works), you could also add the deleted...

Dividing by 11-2

Any number written in our decimal system is made up of powers of 10. For example, 65,321 = 6(10^4) + 5(10^3)+3(10^2) + 2(10)+1. Now if you are interested in knowing whether 65,321 is a multiple of 5 (that is, is exactly divisible by 5), you can tell just by looking at the last digit. That's because 10^4, 10^3, 10^2, and 10 are all divisible by 5, so the last digit on the end gets to cast...