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```        Number of ways of selecting two integers 'a' and 'b' from the set{1,2,3,4,.....5n}, n€N so that a4-b4 is divisible by5?
A)17n2+5n/2    B)15n2-17n/2    C)17n2-5n/2    D)15n2+17n/2
```
7 years ago

## Answers : (1)

147 Points
```										Dear siri
give set {1,2,3,4,.....5n}
case 1:  when intiger a,b, are prime to 5.  then  (a4-1)  and (b4-1)  will be divisible by 5
a4  -b4  =(a4-1)- (b4 -1)
total number of selecting rhe a,b  = 4n(4n-1)/2

case 2 : when a,b are  multiple of 5
total  number of selecting rhe a,b  = n(n-1)/2

so total = 4n(4n-1)/2  + n(n-1)/2
=17n2-5n/2
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```
7 years ago
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