Olympiad Maths Prep

Track / Stage 6 / 306 of 400 #1306 of 2000

Problem 1306

National olympiad, first round
Algebra Difficulty 6.5 Find the answer

Let Q(x)=a0+a1x++anxnQ(x)=a_0+a_1x+\dots+a_nx^n be a polynomial with integer coefficients, and 0ai<30\le a_i<3 for all 0in0\le i\le n.

Given that Q(3)=20+173Q(\sqrt{3})=20+17\sqrt{3}, compute Q(2)Q(2).

Official solution

1. Given the polynomial Q(x)=a0+a1x+a2x2++anxn Q(x) = a_0 + a_1 x + a_2 x^2 + \dots + a_n x^n with integer coefficients ai a_i such that 0ai<3 0 \le a_i < 3 for all 0in 0 \le i \le n , and Q(3)=20+173 Q(\sqrt{3}) = 20 + 17\sqrt{3} , we need to determine Q(2) Q(2) .

2. Since Q(3)=20+173 Q(\sqrt{3}) = 20 + 17\sqrt{3} , we can write:
Q(3)=a0+a13+a2(3)2++an(3)n=20+173 Q(\sqrt{3}) = a_0 + a_1 \sqrt{3} + a_2 (\sqrt{3})^2 + \dots + a_n (\sqrt{3})^n = 20 + 17\sqrt{3}

3. Separate the polynomial into rational and irrational parts:
a0+a2(3)2+a4(3)4+=20 a_0 + a_2 (\sqrt{3})^2 + a_4 (\sqrt{3})^4 + \dots = 20
a13+a3(3)3+a5(3)5+=173 a_1 \sqrt{3} + a_3 (\sqrt{3})^3 + a_5 (\sqrt{3})^5 + \dots = 17\sqrt{3}

4. Simplify the irrational part:
a13+a333+a593+=173 a_1 \sqrt{3} + a_3 3\sqrt{3} + a_5 9\sqrt{3} + \dots = 17\sqrt{3}
a1+3a3+9a5+=17 a_1 + 3a_3 + 9a_5 + \dots = 17

5. Simplify the rational part:
a0+a23+a49+=20 a_0 + a_2 3 + a_4 9 + \dots = 20

6. Since 0ai<3 0 \le a_i < 3 , we need to find coefficients ai a_i that satisfy these equations. We start with the irrational part:
a1+3a3+9a5=17 a_1 + 3a_3 + 9a_5 = 17
Given the constraints, we can try a5=1 a_5 = 1 , a3=2 a_3 = 2 , and a1=2 a_1 = 2 :
2+32+91=2+6+9=17 2 + 3 \cdot 2 + 9 \cdot 1 = 2 + 6 + 9 = 17

7. Now, for the rational part:
a0+3a2+9a4=20 a_0 + 3a_2 + 9a_4 = 20
Given the constraints, we can try a4=2 a_4 = 2 , a2=0 a_2 = 0 , and a0=2 a_0 = 2 :
2+30+92=2+0+18=20 2 + 3 \cdot 0 + 9 \cdot 2 = 2 + 0 + 18 = 20

8. Thus, the polynomial is:
Q(x)=2+2x+0x2+2x3+2x4+x5 Q(x) = 2 + 2x + 0x^2 + 2x^3 + 2x^4 + x^5

9. To find Q(2) Q(2) :
Q(2)=2+22+022+223+224+25 Q(2) = 2 + 2 \cdot 2 + 0 \cdot 2^2 + 2 \cdot 2^3 + 2 \cdot 2^4 + 2^5
Q(2)=2+4+0+16+32+32=86 Q(2) = 2 + 4 + 0 + 16 + 32 + 32 = 86

The final answer is 86 \boxed{86} .

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