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Algebra Difficulty 6.6 National olympiad Prove it Czech Republic

Solve the following system of equations in the domain of real numbers
2x+y=2022,2x + \lfloor y \rfloor = 2022,
3y+2x=2023.3y + \lfloor 2x \rfloor = 2023.
(The symbol a\lfloor a \rfloor denotes the lower integer part of a real number aa, i.e. the greatest integer not greater than aa. E.g. 1.9=1\lfloor 1.9 \rfloor = 1 and 1.1=2\lfloor -1.1 \rfloor = -2.)

Solutions — 2

Solution 1

Since y\lfloor y \rfloor and 20222022 are integers, the equation 2x+y=20222x + \lfloor y \rfloor = 2022 implies that 2x2x is also an integer, so 2x=2x\lfloor 2x \rfloor = 2x. Thus we can eliminate the unknown xx by subtracting the first equation of the system from the second one. We get
3yy=1.(1) 3y - \lfloor y \rfloor = 1. \qquad (1)
Thanks to (1), 3y3y is an integer, so it has (according to its remainder after division by three) one of the forms 3k3k, 3k+13k + 1 or 3k+23k + 2, where kk is an integer. From this it follows that either y=ky = k, or y=k+13y = k + \frac{1}{3}, or y=k+23y = k + \frac{2}{3}, where k=yk = \lfloor y \rfloor. We now discuss these three cases.

* In the case of y=ky = k, (1) becomes 3kk=13k - k = 1 with the non-integer solution k=12k = \frac{1}{2}.

* In the case of y=k+13y = k + \frac{1}{3}, (1) is the equation (3k+1)k=1(3k + 1) - k = 1 with a solution k=0k = 0, which corresponds to y=13y = \frac{1}{3}. The original system of equations is then apparently fulfilled, precisely when 2x=20222x = 2022, i.e. x=1011x = 1011.

* In the case of y=k+23y = k + \frac{2}{3}, (1) is the equation (3k+2)k=1(3k + 2) - k = 1 with a non-integer solution k=12k = -\frac{1}{2}.

Conclusion. The only solution of the given system is the pair (x,y)=(1011,13)(x, y) = (1011, \frac{1}{3}).

Solution 2

The verbal definition attached to the problem formulation tells us that a\lfloor a \rfloor is an integer for which aa\lfloor a \rfloor \le a and at the same time a+1>a\lfloor a \rfloor + 1 > a. So, estimates a1<aaa - 1 < \lfloor a \rfloor \le a are valid for every real number aa. According to them, we get from the first equation of the given system
20222x+y<2023.(2) 2022 \le 2x + y < 2023. \qquad (2)
Similarly, from the second equation follows
20233y+2x<2024.(3) 2023 \le 3y + 2x < 2024. \qquad (3)
We can combine these inequalities in two ways. Combining the second part of (2) with the first part of (3) we obtain 2x+y<20233y+2x2x + y < 2023 \le 3y + 2x, whence from the comparison of the outermost expressions follows y>0y > 0. If we modify the first part of (2) to 20242x+y+22024 \le 2x + y + 2, then together with the second part of (3) we obtain 3y+2x<20242x+y+23y + 2x < 2024 \le 2x + y + 2. This time y<1y < 1 follows from the comparison of the outermost expressions.

Together, we got 0<y<10 < y < 1, so y=0\lfloor y \rfloor = 0 holds. Thanks to this, the first equation of the original system is reduced to the form 2x=20222x = 2022, which is satisfied only for x=1011x = 1011. By inserting it into the second equation, we get 3y+2022=20233y + 2022 = 2023 with the only solution y=13y = \frac{1}{3} which indeed satisfies the condition y=0\lfloor y \rfloor = 0 used in the first equation. The pair (x,y)=(1011,13)(x, y) = (1011, \frac{1}{3}) is therefore the only solution of the given system.

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