I’ve got to make all numbers from 1-100 using only the numbers 1, 9, 7, 8. I must use them all and only once, I can make numbers, (e.g 97 out of 9,7), they can be in any order, and I can use any operation (apart from powers not using 1978). I’ve got 37 and 38 left. Can anyone help me?
This question has had a particular effect on me. I made a program to work on this, in every bit as inefficient a manner as you would expect. Here's what it found...
Using only addition, subtraction, multiplication and division, it found that 45 of the 100 numbers can be computed:
By allowing negation, there were no new results. I guess that makes sense, considering that just winds up multiplying by a difference in reverse order. There's probably some deeper meaning to this that I can't be bothered to dig into.
Combining two sets of digits into a single number, such as taking 1 and 8 to make 18, is called concatenation. Allowing concatenation reaches another 29 answers:
But this doesn't get us all the way there. I was thinking of other potential simple operations and decided to give square root a try. This gives us another 16 answers:
In total, with these operations, we're up to, um... 99 answers. So close! Only 50 remains, and it curiously can't be formed from any combination of the operations explored so far.
For each unary operation in the mix, my software takes exponentially long to compute (it can attach any combination of them to all numbers and binary operations), so I was really pushing my luck by allowing square root and ceiling/floor to play together... But it gave me this:
50 = ⌊(√(1 + 7) + √8) * 9⌋
So I guess that's the full set. Not sure how I feel about letting ceiling and floor participate, since that along with the likes of remainder division always felt a bit destructive to me. I'm also not a fan of using square root since in essence it's just raising to the power of 21, and exponents are apparently not allowed.
I assume this means you can use exponents, but only when you explicitly use the allowed digits. That is, the exponent must be one of the four digits, as in 8^(9-7)-1 = 63, or 1^7+8+9-18, not something squared, using a 2 as the exponent. (Not that I expect that to be very useful.)
This would probably also apply to roots, except that the index 2 doesn't have to be shown for a square root.
So I guess that's the full set. Not sure how I feel about letting ceiling and floor participate, since that along with the likes of remainder division always felt a bit destructive to me. I'm also not a fan of using square root since in essence it's just raising to the power of 21, and exponents are apparently not allowed.
I very much dislike using floor and ceiling in these; did you try using square root without them, to get everything? I think it is standard to allow it.
I did, yeah. Square root failed to account for 38, 49, 93 and 98, which were all matched by floors and ceilings. 50 required the use of both square root and floor.
Allowing exponentiation fares much better: every number except 38, 50 and 93 turns up this way, without ceiling or floor. Please excuse the reverse Polish notation below: it's late and I don't feel like formatting them all at the moment.
I assume this means you can use exponents, but only when you explicitly use the allowed digits. That is, the exponent must be one of the four digits, as in 8^(9-7)-1 = 63, or 1^7+8+9-18, not something squared, using a 2 as the exponent. (Not that I expect that to be very useful.)
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