Let be a positive integer and define , where . Find the value of when the value of is largest.
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A n − 1 A n = n ! 3 0 n + 1 5 n ⋅ 3 0 n − 1 + 1 5 n − 1 ( n − 1 ) !
= n 1 ⋅ 1 5 n − 1 ( 2 n − 1 + 1 ) 1 5 n ( 2 n + 1 )
= n 1 5 ⋅ 2 n − 1 + 1 2 ⋅ 2 n − 1 + 2 − 1
= n 1 5 ( 2 − 2 n − 1 + 1 1 )
= n 3 0 − n ( 2 n − 1 + 1 ) 1 5
Consider which A n − 1 A n will slightly lesser than 1 and slightly larger than 1.
When n = 2 9 , term become 1 . 0 3 4 4 8 . . . − δ , δ is a number very small, actually 2 9 ( 2 2 8 + 1 ) 1 5 = 1 . 9 2 7 e − 9 . so we can neglect δ .
When n = 3 0 , term become 1 − δ , this δ will smaller than before, but even if δ is very small, term will always smaller than 1, slightly.
So we get
A 2 8 A 2 9 > 1
A 2 9 A 3 0 < 1
We know A 2 9 will be largest.