7 (number)

File:En-us-Seven.ogg

Pronunciation of the number 7

The number seven is a natural number that comes after six and before eight. In Roman numerals, it is VII.

Mathematics

In mathematics, the number seven is an odd number and a prime number. It is also a Mersenne prime.

Divisibility Rule

• To test if an integer is divisible by 7 or not,

1. Take off the last digit and multiply it by 2.
2. Subtract the root number from the product .
3. Repeat the second step again and again until it is clearly that the number is divisible by 7 or not.

Example

918785

1. 5×2=10, 91878-10=91868
2. 8×2=16, 9186-16=9170
3. 0×2=2, 917-0=917
4. 7×2=14, 91-14=77

77 is divisible by 7, so the original number 918785 is also divisible by 7.

Proof: Pretend [math]\displaystyle{ a }[/math] is the root number and [math]\displaystyle{ b }[/math] is the last digit. Then [math]\displaystyle{ 10a+b }[/math] is the whole number. If the whole number ([math]\displaystyle{ 10a+b }[/math]) is divisible by 7, [math]\displaystyle{ 3a+b }[/math] is also a multiple of 7. And [math]\displaystyle{ (3a+b)-7b= 3a-6b }[/math] also divide 7. That proves [math]\displaystyle{ 3a-6b }[/math] is divisible seven, since 3 and 6 are both multiples of 3. So [math]\displaystyle{ a-2b }[/math] is also divisible by 7.

In the world

• There are seven colors in the rainbow.
• The atomic number of nitrogen.
• The number of horizontal rows of elements in the periodic table.
• Seven gates of heaven (QUR'AN 15th chapter AL-HIJR verse 44)915:44)
• Seven days in a week
• Seven seas and seven continents.
• Seven major parts of the human body (2 legs, 2 hands, trunk, neck and head)

References

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