Answer:
you need to show the number is prime
Step-by-step explanation:
To show 2^7 -1 is prime, you have to show it is not divisible by any prime less than its square root. That root is approximately 2^(7/2) = 2^3×√2 ≈ 11.3.
The primes of in that range are 2, 3, 5, 7, 11. The prime we're testing is ...
2^7 -1 = 128 -1 = 127
The sum of digits is 10, which is not a multiple of 3: not divisible by 3.
The units digit is 7, not 0 or 5: not divisible by 5.
The test for divisibility by 7 is to subtract twice the units digit from the rest of the number: 12 -2(7) = -2. This is not 0 or a multiple of 7, so 127 is not divisible by 7.
We know that 11² = 121, so 127 is not divisible by 11.
All of the tests for primality pass, so 2^7 -1 is a Mersenne prime.
_____
Additional comment
It is known that 2^n -1 being prime requires n to be prime. (The converse is not true: 2^11 -1 = 2047 = 23×89 is not prime.) The discovery of Mersenne primes is an ongoing process involving thousands of computers around the world. Only about 50 such primes are known.
You will notice that we have made use of some divisibility rules in order to simplify the process of dividing by small numbers. These can be helpful in many situations, so can be useful to learn.
