Why is 1 not a Prime Number?

Is 1 a Prime Number?

Friends, in one of the post where I have described ‘Prime’ and ‘Composite’ Numbers, one of the curious visitor have asked me a very logical question. I will quote that question for your reference –
Text from Previous post-

“Prime and Composite : Any integer which is divisible by 1 and itself only is called a prime number.
unquote

quote
N.B.: 1 is not a prime number.”

Question

Could you explaine what is the creteria thar excludes 1 from the list of prime numbers?
a) 1 is integer
b) 1 is divisible by 1 and itself (1)
Since anybody in the past has declared that 1 is not prime number, why we should follow this without thinking and contravene the general rule for prime numbers?
Is 1 as a figure is something which has come from the thin air. It is and always will be an integer. The criteria for 2 are the same – divisible by 1 and itself. And for all prime numbers.
Most probably the 1 is “guilty” because with 1 starts the series on numbers (natural, odd or prime). Suppose 2 was the beginning of the series. Should we ignore 2, because series starts with 2?

Decimal Fraction Rules

Multiplication of a decimal Fraction by a Power of 10: Rule: Shift the decimal point to the right by as many places of decimal as   is the power of 10.

Multiplication of Decimal fractions:- Rule :- Multiply the given numbers considering them without the  decimal  point. Now, in the product, the decimal point is marked off to obtain as many places of decimal as is the sum of the number of decimal in the given numbers.

Dividing a Decimal fraction By a Counting Number

Rule: – Divide the given number without considering the decimal point  by the given counting   number. Now, in the quotient, put the decimal point to give as many places of decimal as are there in  the dividend.

There are 9 coins.

There are 9 coins. 8 are of 1 gm and 1 is of 2 grams. How will you find out the heavier coin in minimum number of weighing and how many weighing it will need?

Solve a Dilemma

What is wrong with this proof?

2 = 1

a = b

a2 = ab

a2 – b2 = ab – b2

(a + b) (a – b) = b (a – b)

a + b = b

2b = b

2 = 1

A Puzzle Of Cultural Groups

My club has five cultural groups. They are literary, dramatic, musical, dancing and painting groups. The literary group meets every other day, the dramatic every third day, the musical every fourth day, the dancing every fifth day and the painting every sixth day. The five groups met, for the first time on the New Year’s day of 1975 and starting from that day they met regularly according to schedule.

Now, can you tell how many times did all the five meet on one and the same day in the first quarter? Of course the New Year’s day is excluded.

One more question-were there any days when none of the groups met in the first quarter and if so how many were there?

Entrance Test

As an entrance test for a particular university, you are given a corked bottle with a very small coin in it. Your task is to remove the coin without taking the cork out of the bottle or breaking the glass, or boring a hole in the cork or glass.

How do you pass the test and get the coin out?

Vedic Multiplication by 9, 99, 999 and so on

When any number has to be multiplied by a series of 9s, like 9, 99, 999, 9999 and so on than we can apply this very simple vedic maths technique to increase your speed of calculation.

Multiplication with 9/ 99 / 999 and so on.

we know, 789 × 999 = 788,211

You will get the answers in two parts,

• The left hand side of the answer: subtract 1 from 789, which is 788
• The right hand side of the answer subtract 789 from 1000 = 1000-789= 211

Thus, 999 x 789 = 789-1   |  1000-789 = 788, 211 (answer)

{for the right hand side of the answer, 789 should be subtracted from (999+1)}

or,  99999 x 78 = 78-1   | 100000 – 78

= 7799922

{78 should be subtracted from (99999+1)}

Another example:

1203579 × 9999999 = 1203579-1   | 10000000- 1203579

=120357887964 21

Number in red is 1 less than 1203579. Number in blue is (10000000-1203579). Hence the answer.

This method has to be altered a little bit when number of 9s are lessers than the number of digit in the divisor.

1432  x 9 = 1432 (10 – 1) = 14320 – 1432 = 12888

So for multiplication with 9, put a zero after that number and subtract the number itself from that.

Likewise for 99 put two zeroes after that number .

3256 x 99 = 325600 – 3256 =  322344

Eye Spy

Which is Wrong?

Click on image to enlarge

Astral Maths

Look at the shape below and answer the following questions on it.

1. How many triangles are there in the diagram?
2. How many rectangles are there in the diagram?
3. How many hexagons can you find?
4. Deduct the sum of the numbers in the rectangles from the sum of the numbers in the triangles.