Maths Olympiad Prep

Track / Stage 5 / 175 of 400 #775 of 1964

Problem 775

AIME late
Number theory Difficulty 5.4 Find the answer

4.67 Euler's conjecture was disproved by American mathematicians in 1960, who confirmed the existence of a positive real number nn, such that
1335+1105+845+275=n5, 133^{5}+110^{5}+84^{5}+27^{5}=n^{5},

Find the value of nn.

A number or a short expression. Spacing, $ signs and \frac vs / are all fine.

Official solutions — 2

Solution 1

[Solution]Obviously n134n \geqslant 134. On the other hand,
n5=1335+1105+845+275<1335+1105+(84+27)5<31335<312510241335=(5×1334)5=(16614)5, \begin{aligned} n^{5} & =133^{5}+110^{5}+84^{5}+27^{5} \\ & <133^{5}+110^{5}+(84+27)^{5} \\ & <3 \cdot 133^{5} \\ & <\frac{3125}{1024} \cdot 133^{5} \\ & =\left(\frac{5 \times 133}{4}\right)^{5} \\ & =\left(166-\frac{1}{4}\right)^{5}, \end{aligned}

Therefore, n166n \leqslant 166.
Since a5a^{5} has the same last digit as aa, the last digit of nn is the same as the last digit of 3+0+4+73+0+4+7, which means the last digit of nn is 44. Thus, the possible values of nn are 134,144,154134, 144, 154, or 164164.

Noting that 84 and 27 are both multiples of 3, and 1335133^{5} leaves a remainder of 1 when divided by 3, while 1105110^{5} leaves a remainder of 2 when divided by 3, it follows that 1335+1105+845+275133^{5}+110^{5}+84^{5}+27^{5} is a multiple of 3, meaning nn is a multiple of 3.

Among 134,144,154,164134, 144, 154, 164, only 144 is a multiple of 3, so nn can only be 144.
Upon verification, 1445=1335+1105+845+275144^{5}=133^{5}+110^{5}+84^{5}+27^{5}. Therefore, the value of nn is 144144.

Solution 2

[Solution] Clearly, n134n \geqslant 134. On the other hand,
n5=1335+1105+845+275<1335+1105+(84+27)5<31335<312510241335=(5×1334)5=(16614)5\begin{array}{l} n^{5}= 133^{5}+110^{5}+84^{5}+27^{5} \\ <133^{5}+110^{5}+(84+27)^{5} \\ <3 \cdot 133^{5} \\ <\frac{3125}{1024} \cdot 133^{5} \\ =\left(\frac{5 \times 133}{4}\right)^{5} \\ =\left(166 \frac{1}{4}\right)^{5} \end{array}

Therefore, n166n \leqslant 166.
Since a5a^{5} has the same last digit as aa, the last digit of nn is the same as the last digit of 3+0+4+73+0+4+7, which means the last digit of nn is 44. Thus, the possible values of nn are 134,144,154134, 144, 154, or 164164.

Noting that 84 and 27 are both multiples of 3, and 1335133^{5} leaves a remainder of 1 when divided by 3, while 1105110^{5} leaves a remainder of 2 when divided by 3, it follows that 1335+1105+845+275133^{5}+110^{5}+84^{5}+27^{5} is a multiple of 3, meaning nn is a multiple of 3.

Among 134,144,154,164134, 144, 154, 164, only 144 is a multiple of 3, so nn can only be 144.

Upon verification, 1445=1335+1105+845+275144^{5}=133^{5}+110^{5}+84^{5}+27^{5}. Therefore, the value of nn is 144144.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.