Olympiad Maths Prep

Track / Stage 6 / 52 of 400 #1052 of 2000

Problem 1052

National olympiad, first round
Algebra Difficulty 6.1 Find the answer

How many real numbers are roots of the polynomial x937x82x7+74x6+x437x32x2+74x?x^9 - 37x^8 - 2x^7 + 74x^6 + x^4 - 37x^3 - 2x^2 + 74x?

Official solution

1. Given the polynomial:
P(x)=x937x82x7+74x6+x437x32x2+74x P(x) = x^9 - 37x^8 - 2x^7 + 74x^6 + x^4 - 37x^3 - 2x^2 + 74x
We need to determine the number of real roots.

2. The polynomial can be factored as:
P(x)=(x437x32x2+74x)(x5+1) P(x) = (x^4 - 37x^3 - 2x^2 + 74x)(x^5 + 1)

3. Further factorization of x437x32x2+74xx^4 - 37x^3 - 2x^2 + 74x:
x437x32x2+74x=x(x337x22x+74) x^4 - 37x^3 - 2x^2 + 74x = x(x^3 - 37x^2 - 2x + 74)

4. The polynomial x32xx^3 - 2x can be factored as:
x32x=x(x22)=x(x2)(x+2) x^3 - 2x = x(x^2 - 2) = x(x - \sqrt{2})(x + \sqrt{2})

5. Therefore, the factorization of the original polynomial becomes:
P(x)=x(x2)(x+2)(x37)(x5+1) P(x) = x(x - \sqrt{2})(x + \sqrt{2})(x - 37)(x^5 + 1)

6. The roots of x(x2)(x+2)(x37)x(x - \sqrt{2})(x + \sqrt{2})(x - 37) are:
x=0,±2,37 x = 0, \pm \sqrt{2}, 37
These are four distinct real roots.

7. The polynomial x5+1x^5 + 1 can be factored using the sum of cubes:
x5+1=(x+1)(x4x3+x2x+1) x^5 + 1 = (x + 1)(x^4 - x^3 + x^2 - x + 1)
The factor x+1x + 1 has a real root:
x=1 x = -1
The factor x4x3+x2x+1x^4 - x^3 + x^2 - x + 1 has no real roots because its discriminant is negative, indicating no real solutions.

8. Combining all the real roots, we have:
x=0,±2,37,1 x = 0, \pm \sqrt{2}, 37, -1

9. Therefore, the total number of real roots is:
5 \boxed{5}

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