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Number theory Difficulty 6.2 National olympiad Prove it Mongolia

In how many manner can the number 32014\frac{3}{2014} be represented in the form
1p+1q, p,qN \frac{1}{p} + \frac{1}{q}, \ p, q \in \mathbb{N}

Solution

32014=1p+1q3pq=2014(p+q)=21953(p+q).\frac{3}{2014} = \frac{1}{p} + \frac{1}{q} \Rightarrow 3pq = 2014(p+q) = 2 \cdot 19 \cdot 53(p+q).

i) Consider the case 21953p2 \cdot 19 \cdot 53 \mid p. Setting p=21953rp = 2 \cdot 19 \cdot 53r we get 3rq=21953r+qq=21953r3r13rq = 2 \cdot 19 \cdot 53r + q \Rightarrow q = \frac{2 \cdot 19 \cdot 53r}{3r-1}. Since qNq \in \mathbb{N}, 3r121953r3r-1\mid 2 \cdot 19 \cdot 53r, (r,3r1)=13r1(r, 3r-1) = 1 \Rightarrow 3r-1 may take values 2, 38, 53, 1007. Therefore
3r1=2r=1,3r1=38r=13,3r1=53r=18,3r1=1007r=336 \begin{align*} 3r - 1 &= 2 \Rightarrow & r &= 1, \\ 3r - 1 &= 38 \Rightarrow & r &= 13, \\ 3r - 1 &= 53 \Rightarrow & r &= 18, \\ 3r - 1 &= 1007 \Rightarrow & r &= 336 \end{align*}
and corresponding 4 pairs (p,q)(p, q) are (2014,1007)(2014, 1007), (26182,689)(26182, 689), (36252,684)(36252, 684), (676704,672)(676704, 672).

ii) Consider the case 219p2 \cdot 19 \nmid p and 53p53 \nmid p. Setting p=219rp = 2 \cdot 19 \cdot r we get 3219rq=21953(219r+q)3 \cdot 2 \cdot 19 \cdot rq = 2 \cdot 19 \cdot 53(2 \cdot 19r + q). Let q=53sq = 53s. Consequently, s=219r3r53s = \frac{2 \cdot 19r}{3r - 53}, (r,3r53)=13r53219r(r, 3r - 53) = 1 \Rightarrow 3r - 53\mid 2 \cdot 19r. Therefore
3r53=1r=18,3r53=19r=24 \begin{align*} 3r - 53 &= 1 \Rightarrow & r &= 18, \\ 3r - 53 &= 19 \Rightarrow & r &= 24 \end{align*}

iii) Consider the case 253p2 \cdot 53 \nmid p and 19p19 \nmid p. From here follows p=253r\Rightarrow p = 2 \cdot 53r and setting q=19sq = 19s we get s=253r3r19s = \frac{2 \cdot 53r}{3r - 19}. It implies 3r19253r3r - 19 \mid 2 \cdot 53r. 3r19=2r=73r - 19 = 2 \Rightarrow r = 7
3r19=53r=24.3r - 19 = 53 \Rightarrow r = 24.
If r=7r = 7 then p=2753p = 2 \cdot 7 \cdot 53, q=71953(p,q)=(742,7049)q = 7 \cdot 19 \cdot 53 \Rightarrow (p, q) = (742, 7049)
If r=24r = 24 then p=22453p = 2 \cdot 24 \cdot 53, q=1948(p,q)=(2544,912)q = 19 \cdot 48 \Rightarrow (p, q) = (2544, 912)

iv) Consider the case 1953p19 \cdot 53 \nmid p and 2p2 \nmid p. Setting p=1953rp = 19 \cdot 53r, q=2sq = 2s we get s=1953r3r2s = \frac{19 \cdot 53r}{3r-2}. Therefore 3r21953r3r - 2 \mid 19 \cdot 53r.
3r2=1r=1;3r - 2 = 1 \Rightarrow r = 1;
3r2=19r=73r-2=19 \Rightarrow r=7. Corresponding pairs are (p,q)=(1007,2014);(7049,742)(p,q) = (1007, 2014); (7049, 742).

Thus there are 6 possibilities: 32014=12014+11007=126182+1689=\frac{3}{2014} = \frac{1}{2014} + \frac{1}{1007} = \frac{1}{26182} + \frac{1}{689} =
1676704+1672=136252+1684=1912+12544=17049+1742. \frac{1}{676704} + \frac{1}{672} = \frac{1}{36252} + \frac{1}{684} = \frac{1}{912} + \frac{1}{2544} = \frac{1}{7049} + \frac{1}{742}.

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