- #1
hellraiser
Hi. We are doing permutations and combinations in class and we were given some formulas without proof to remember. I was able to derive most of them but was unable to derive 3 of them. But I would like to see how do I derive them for sake of fun (also if I forget them what will I do. :) ).
1. A number is expressed in the form of product of it's prime factors
N = a^x b^y c^z etc.
Then the number N can be resolved as a product of 2 factors in how many ways?
[Ans : If N is not a perfect square 0.5(x+1)(y+1)(z+1)
If N is a perfect square 0.5[(x+1)(y+1)(z+1)+1]
]
All I can think is to take it to be of this form xy = N.
Then what do I do next.
2. This one is similar to first. The number of ways in which a composite number N can be resolved into 2 factors which are co prime to each other is 2^(n-1) , where n is number of different factors in N (eg: as in last case n=x+y+z)
3. Number of ways of arranging n differnet objects in r boxes where arrangement matters within a box
[(n+r-1)!] / [(r-1)!]
Any help?
1. A number is expressed in the form of product of it's prime factors
N = a^x b^y c^z etc.
Then the number N can be resolved as a product of 2 factors in how many ways?
[Ans : If N is not a perfect square 0.5(x+1)(y+1)(z+1)
If N is a perfect square 0.5[(x+1)(y+1)(z+1)+1]
]
All I can think is to take it to be of this form xy = N.
Then what do I do next.
2. This one is similar to first. The number of ways in which a composite number N can be resolved into 2 factors which are co prime to each other is 2^(n-1) , where n is number of different factors in N (eg: as in last case n=x+y+z)
3. Number of ways of arranging n differnet objects in r boxes where arrangement matters within a box
[(n+r-1)!] / [(r-1)!]
Any help?