# 2013中国女子数学奥林匹克试题及其解答

2013 中国女子数学奥林匹克试题及其解答

1.证明（张云华）

1.证明（mavropnevma ）Let and

,

. The region and

is the triangle . But is for (equality

,

/>
, thus the triangles of largest possible area are

(equality for disallowed). is for ).

, for

EDIT. Referring to the next post - it is interesting to figure out the envelope of the lines is the parabola functions for a fixed , with triangular area , but in some way it is irrelevant, since the problem fixed on the parabola for some . , and the

being delimited by the tangent to that parabola at

1.证明（kunny） parabola Which touches The domain points at is envelope of the lines .

, thus the

of the family of the lines swept by .

is shown by the shaded region, excluded two

Edit:I was misreading the context problem, thank you for pointing put it, mavropnevma.

I have just attached another figure.Needless to say, mavropnevma's solution is perfect. P.S.I remember that the similar problem original problem has been posed in 1970's in Tokyo University entrance exam/Science. According to my memory, let region be the maximum area of any triangle which is involved in the to find the extrema of .

in original problem, then draw the graph of

Here is the similar problem posed in Tokyo University entrance exam/Science, second round, 1978

In the

-plane, let

be the part which is correspond to .

of the parabola

,

that is to say,

Let the tangent Line of the line Let . at .

at

intersects with the line

at

and intersects with

. We are to consider the questions as below in the range of

(1) Let denote the area of triangles of such that .

by

, respectively. Find the range

(2) Let Note :

be the domain enclosed by line segments contains line segments and .

and

.

Let

be the maximum area of the triangle with a vertex and draw the graph, then find the exterme value.

which is contained in

.

Find the function

Note

:

A

function

has

local

minimum

(or

local

maximum)

at

a

point ,

, which means for all points

which is closed to

holds. We call local maximum, local minimum as extreme value.

2.证明(Luis Gonzá lez)Let of touching at

and Clearly but since and

Let be the incircle are homothethic with incircles ( is

are symmetric about the midpoint of and

the M-excircle of MAB), it follows that with corresponding cevians 2.证明(Andrew64)As shown in the figure below. is the intersection of and .

are homothetic

Therefore Attachments:

,

,

meet at the same point

.

link17.jpg [ 37.23 KiB | Viewed 52 times ] 3.证明(mavropnevma)Since it is irrelevant which persons of the same gender know each other, we may assume there ore none such, and consider the bipartite graph having the left shore made of the boys and the right shore made of the girls, with an edge connecting a boy and a girl if they know each other. The condition means does not contain any induced cycle of length , and the requirement is to show the number of edges satisfies .

Thus it is an extremal graph theory question, for bipartite

graphs with forbidden

's; by

symmetry we should also have Denote by

. the set of girls that each and

the set of girls that each knows exactly one boy, and by and

knows more than one boy; take

. We obviously have

. Let us count the number boys, and one girl knows both

of objects . For each of the

, where

is a girl,

are distinct there is at most

doubletons

knowing them both (by the condition), so

. Moreover, by pigeonhole

we have

.

On

the

other

hand,

we

have

, by Jensen's inequality. We thus need , so

. Finally, we get .

For equality to be reached we obviously need , namely for each pair of boys having exactly one girl knowing both of them; and then we need .

3.证明(crazyfehmy)Another solution: Consider the bipartite graph where there are girls boys and denote the girls by 's and boys by 's as vertices. Let denote the number of edges from the vertex the girl to set . If and is connected to some

and

and

then for any

must not connected to both

. Now let us count such

pairs. For every girl

there are

many pair of edges. Since all such edge pairs must be

distinct for all girls, and since there are at most

such pairs, we have

or equivalently Now assume that or . Then we have are greater than or equal to and are

and we need to show that Since for we have

. and hence .

Now by Cauchy-Schwarz inequality we have

and if

, we have

and we need to show that

. Assume

. Then we have

which means that

which is a contradiction. So, we are done.

4.解 （crazyfehmy） Let

be called a nice pair if

satisfies the conditions

stated in the problem. Firstly, we shall prove a lemma:

If

is a nice pair then

for all integers

and

.

Proof: Let

or

and

divides

for some integers such that

and and

. Then we can find another pair

. (The proof is easy) Then consider the system

and Remainder Theorem this system has a solution an element of

and

. By the Chinese such that both and is is not a nice pair because is divisible by .

which means that

Now, we shall show that if solution for all integers

or

then

has a such that

. For the proof assume there exists an integer

has no solution in integers. Then it is easy to see that has also no solution for all integers which are not divisible

by

. Now take

and consider the numbers

. For all of these numbers,

we have no solution and there are (take ) and there are

such numbers. Since all square residues have solutions square residues modulo including zero, this means that for the number we have a

all nonsquare residues

, the equation must have no solution. However, for so for the number

is not a square residue but solution which is a contradiction. For

is not a square residue but

has a solution since obtain a contradiction so the second lemma is also proved.

is a square residue. Hence we again

Now, since

can take every value modulo and if

and

we must have

and

and also we must have

is a nice pair. So,

can take only three

values. We will consider each case separately:

If

then we must have

which means

If means If means

then we must have

which

then we must have

which

If

we

count

these

possibilities, .

if

i

am

not

wrong,

we

get

4.解（dinoboy）First, remark that it suffices for For modulo For simply note that we require or we

to be injective modulo . require for some

and

.

modulo

that

Now, what values can values modulo

take modulo

? It is a simple exercise to show all and then as can be

can be obtained (just express it as

we can transform the problem to what values modulo expressed as works out is if

, which is known to be all of them). Therefore the only way this problems and .

Therefore we simply require that of modulo , 10 modulo .

and and modulo

. For each value of

there are

values

so the answer should simply be

Note:

To

show

takes

all

values

modulo

without

relying

on

is not hard, but I'm lazy and felt like reducing it to an already solved problem.

4.解（yunxiu） hence

，so satisfies the problem.

.

6.解（crazyfehmy）If since has always a solution

let

then is equivalent to

satisfies which and since is odd. be the and let

in the set

for all

Now we will show that if elements of the set

then the condition does not satisfy. Let

. Consider the sums

. Since are also different modulo

's are different modulo

, the numbers

's

. On the other hand, none of

's can be equivalent to

modulo is a

because otherwise we would have two equivalent terms. Hence permutation of and by adding up these which equations we

get

means

. Now do the same procedure for all . . Then we have Let

's to get and

. So, we have

many numbers equivalent to each other modulo . However, we know that there are many numbers modulo which are all equivalent to each other modulo . Hence in order for 's to be different modulo . So Hence all possible , we must have and we are done. values are which means that . and hence

7.证明 （Luis Gonzá lez）Let then

cut

again at

Since

is the exsimilicenter of of

is midpoint of the arc

bisects externally is midpoint of the arc of is external bisector of and Note that is a Thebault circle of the cevian of externally tangent to its circumcircle By Sawayama's lemma passes through its C-excenter is C-excenter of is M-isosceles, i.e. circumcenter of 7.证明（Andrew64）As shown in the figure. Let be the intersection of and . It's fairly obvious So we have , and So Thus Consequently . , and , are concyclic. Hence is

Namely Attachments:

is the bisector of

.

link18.jpg [ 31.39 KiB | Viewed 93 times ] 8.证明（duanby） hint:(a-b)(c-b)(a-d)(c-d) in detail: product (a-b)(c-b)(a-d)(c-d) for every a,c be the number on , b,d be the number on for point x,y if they are not ajjectent then in the product, it will occur twice, if it's ajjectent it's appears only once, and also chick the point that are on and then we get it. iampengcheng1130 2013 中国女子数学奥林匹克第 7 题的解答

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