7 (number)

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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 that product from the root number (the integer without the last digit).
3. Repeat the second step again and again until it is clear 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=0, 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 $a$ is the root number and $b$ is the last digit. Then $10a+b$ is the whole number. If the whole number ($10a+b$) is divisible by 7, $3a+b$ is also a multiple of 7. And $(3a+b)-7b=3a-6b$ is also a multiple 7. That proves $3a-6b$ is divisible by seven, since 3 and 6 are both multiples of 3. So $a-2b$ 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.
• The seven sacraments of the Catholic church.
• Seven gates of heaven (QUR'AN 15th chapter AL-HIJR verse 44)915:44).
• Seven days in a week.
• Seven seas and seven continents.
• Cristiano Ronaldo wears the number 7, widely known as CR7.
• The number 7 is often considered as luck in Western culture.