Find the smallest positive integer n ≥ 2 such that n cannot be expressed in the form a b + c , where a , b and c are single digit positive integers.
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what is the step of difference between squares ???? ,, can you explain it
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Sorry, I didn't specify they were consecutive (updated now). So the differences are; 2 2 − 1 2 = 3 ; 3 2 − 2 2 = 5 ; 4 2 − 3 2 = 7 ; 5 2 − 4 2 = 9 ; 6 2 − 5 2 = 1 1 ; and so on. If the difference is greater than 9 then you cannot use the form a b + c to reach that number where a b is the smaller square (but as you see with our first possible answer we come across there may a solution using a different power)
If b=1, we get numbers............. 2...to..10
If a=2........b=1 we get ...............3...to..11
.....................=2......."......................5...to..13
.....................=3......."......................9...to..17
.....................=4......."....................17...to..25
Next is 32+1=33 but we got only 25. So going for other power
...a=5...........=2......."....................26...to..34
...a=2...........=5......."....................33...to..41
Next 2^6 stars with 65, 3^3 =27 and we get.....28...to...36
So next would be 6^2=36........................""..........37...to...45
There is no way to get 46.
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It might not be the best solution, but here is how I did it:
Given that c can be anything from 1 to 9 I started with the premise that the number I was looking for needed to be either a b itself or in the range where the difference between consecutive squares is greater than 9 .
This first occurs between a = 5 and a = 6 , where the difference between the squares is 6 2 − 5 2 = 1 1 . We can add 1 0 to 5 2 to get our first possible answer, 3 5 . You can quickly see though that 35 can be obtained by either 3 3 + 8 or 2 5 + 3 so that rules that one out. 36 can be also obtained a similar way.
So now we move up to the next range, between a = 6 and a = 7 . Adding 1 0 to 6 2 gets us 4 6 , which you can validate pretty quickly as out of range of both 3 3 and 2 5 and all other powers available.