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maths

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What is the sum of all integer values of n satisfying 1≤n≤100, such that n2−1 is a product of exactly two distinct prime numbers?

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    so we need to look at the number before a perfect square and see if it is a prime or not

    4-1 = 3 , 1x3 , 1 is not considered prime
    9-1 = 8 more than 1 pair of factors
    16-1 = 15 --> 3x5 , both prime ✔
    25-1 = 24 lots of pairs
    36-1 = 35 --> 5x7 , both prime ✔
    49-1 = 48 lots
    64-1 = 63 lots
    81 - 1 = 80 lots
    100-1 = 99 lots

    so the only n values are 4 and 6
    and their sum is 10

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