Maths-
General
Easy

Question

The number 111…1 (91 times) is a/an

  1. Even number    
  2. Prime number    
  3. Not prime    
  4. None of these    

The correct answer is: Not prime


    Let S equals 1 plus 10 plus 10 to the power of 2 end exponent plus... plus 10 to the power of 90 end exponent
    equals fraction numerator 1 bullet left parenthesis 10 to the power of 91 end exponent minus 1 right parenthesis over denominator 10 minus 1 end fraction equals fraction numerator open parentheses 10 to the power of 13 end exponent close parentheses to the power of 7 end exponent minus 1 over denominator 10 to the power of 13 end exponent minus 1 end fraction cross times fraction numerator 10 to the power of 13 end exponent minus 1 over denominator 10 minus 1 end fraction
    equals open square brackets open parentheses 10 to the power of 13 end exponent close parentheses to the power of 6 end exponent plus open parentheses 10 to the power of 13 end exponent close parentheses to the power of 5 end exponent plus open parentheses 10 to the power of 13 end exponent close parentheses to the power of 4 end exponent plus... plus 1 close square brackets cross times left parenthesis 10 to the power of 12 end exponent plus 10 to the power of 11 end exponent plus... plus right parenthesis
    therefore blankIt is the product of two integers and hence not prime.

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