Maximum

Algebra Level 5

Let n n be a positive integer and define A n = 3 0 n + 1 5 n n ! A_{n}=\dfrac{30^n+15^n}{n!} , where n ! = 1 × 2 × 3 × . . . × n n!=1 \times 2 \times 3\times...\times n . Find the value of n n when the value of A n A_{n} is largest.


The answer is 29.

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1 solution

Kelvin Hong
Jun 8, 2017

A n A n 1 = 3 0 n + 1 5 n n ! ( n 1 ) ! 3 0 n 1 + 1 5 n 1 \frac{A_n}{A_{n-1}}=\frac{30^n+15^n}{n!} \cdot \frac{(n-1)!}{30^{n-1}+15^{n-1}}

= 1 n 1 5 n ( 2 n + 1 ) 1 5 n 1 ( 2 n 1 + 1 ) = \frac{1}{n} \cdot \frac{15^n (2^n+1)}{15^{n-1} (2^{n-1}+1)}

= 15 n 2 2 n 1 + 2 1 2 n 1 + 1 =\frac{15}{n} \cdot \frac{2 \cdot 2^{n-1} +2-1}{2^{n-1}+1}

= 15 n ( 2 1 2 n 1 + 1 ) =\frac{15}{n} \bigg( 2-\frac{1}{2^{n-1}+1} \bigg)

= 30 n 15 n ( 2 n 1 + 1 ) =\frac{30}{n}-\frac{15}{n(2^{n-1}+1)}

Consider which A n A n 1 \frac{A_n}{A_{n-1}} will slightly lesser than 1 and slightly larger than 1.

When n = 29 n=29 , term become 1.03448... δ 1.03448...- \delta , δ \delta is a number very small, actually 15 29 ( 2 28 + 1 ) = 1.927 e 9 \frac{15}{29(2^{28}+1)}=1.927e-9 . so we can neglect δ \delta .

When n = 30 n=30 , term become 1 δ 1- \delta , this δ \delta will smaller than before, but even if δ \delta is very small, term will always smaller than 1, slightly.

So we get

A 29 A 28 > 1 \frac{A_{29}}{A_{28}}>1

A 30 A 29 < 1 \frac{A_{30}}{A_{29}}<1

We know A 29 \boxed {A_{29}} will be largest.

Good solution! Now I'll delete mine :)

Steven Jim - 4 years ago

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Don't need to do that... hahaha

Kelvin Hong - 4 years ago

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