Maths Olympiad Prep

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, 2022

Number theory Difficulty 2.1 Junior Prove it Canada

Find the three ordered pairs of integers (a,b)(a,b) with 1<a<b1<a<b and ab=2022ab=2022.
Suppose that cc and
dd are integers with c>0c>0 and d>0d>0 and 2c+12d+1=117\dfrac{2c+1}{2d+1}=\dfrac{1}{17}. What is
the smallest possible value of dd?
Suppose that pp, rr and tt are real numbers for which (px+r)(x+5)=x2+3x+t(px+r)(x+5)=x^2+3x+t is true for all real
numbers xx. Determine the value of
tt.

Solution

Factoring, $2022 = 2 \cdot 1011 = 2
3\cdot 3 \cdot 337$. (It turns out that 337 is a prime number,
though this fact is not needed here.)

Therefore, 2022=210112022 = 2 \cdot 1011 and
2022=36742022 = 3 \cdot 674 and 2022=63372022 = 6 \cdot 337.

Thus, the three ordered pairs are $(a,b) =
(2, 1011), (3, 674), (6, 337)$.
Manipulating algebraically, the following equations are
equivalent: 2c+12d+1=11717(2c+1)=2d+134c+17=2d+134c+16=2dd=17c+8\begin{aligned} \dfrac{2c+1}{2d+1} & = \dfrac{1}{17}\\ 17(2c+1) & = 2d + 1 \\ 34c + 17 & = 2d + 1 \\ 34c + 16 & = 2d \\ d & = 17c + 8\end{aligned} Since cc is an integer with c>0c > 0, then c1c \geq 1, which means that 17c+82517c + 8 \geq 25.

Therefore, the smallest possible value of dd is d=25d=25.

Note that, when d=25d = 25, we obtain
c=1c=1 and so $2c+12d+1=351=117$.\$\dfrac{2c+1}{2d+1} = \dfrac{3}{51} = \dfrac{1}{17}\$.
Solution 1

When x=5x = -5, the left side of the
equation equals 0.

This means that when x=5x = -5, the
right side of the equation must equal 0 as well.

Thus, (5)2+3(5)+t=0(-5)^2 + 3(-5) + t = 0 and so
2515+t=025 - 15 + t = 0 or t=10t = -10.

Solution 2

Expanding the left side, we obtain (px+r)(x+5)=px2+rx+5px+5r(px+r)(x+5) = px^2 + rx + 5px + 5r Since
this is equal to x2+3x+tx^2 + 3x + t for
all real numbers, then the coefficients of the two quadratic expressions
must be the same.

Comparing coefficients of x2x^2, we
obtain p=1p = 1.

This means that x2+rx+5x+5r=x2+3x+tx^2 + rx + 5x + 5r = x^2 + 3x + t Comparing coefficients of xx, we obtain r+5=3r+5 = 3 and so r=2r = -2.

This means that x2+3x10=x2+3x+tx^2 + 3x - 10 = x^2 + 3x + t Comparing constant terms, we obtain t=10t = -10.

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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.