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Math Problems Set 8

Discussion: Oct. 25, Oct. 27 (on polynomials and floor functions) The name after
the problem is the designated writer of the solution of that problem . (Beth, Nicholas, and
Frank are exempted this week)

Discussion Problems

1. (a) Factor the polynomial x8 +98x4 +1 into two factors with integer (not necessarily
real) coefficients.
(b) Find the remainder on dividing x 100 - 2x51 + 1 by x2 - 1. (Shelley)
(Hint: (a) 98 = 100 - 2; (b) Bezout’s theorem)

2. If and are the zeros of the polynomial x2 - 6x + 1, then for every nonnegative
integer n, is an integer and not divisible by 5. (Derek) (Hint: how about
induction?)

3. (VA 1982) Let p(x) be a polynomial of the form p(x) = ax2 + bx + c, where a, b and
c are integers, with the property that 1 < p(1) < p(p(1)) < p(p(p(1))). Show that
a ≥ 0. (Brett) (Hint: by contradiction)

4. (VA 1987) A sequence of polynomials is given by , for
n ≥ 0, where and, for n ≥ 0, . Denote by and
the roots of , with . Find and . (Ben) (Hint:
think about and .)

5. (VA 1991) Prove that if α is a real root of (1-x2)(1+x+x2+· · ·+xn)-x = 0 which lies
in (0, 1), with n = 1, 2, · · · , then is also a root of .
(Lei) (Hint: use

6. (VA 1996) Let , i = 1, 2, 3, 4, be real numbers such that .
Show that for arbitrary real numbers , i = 1, 2, 3, the equation
has at least one real root which is on the interval
-1 ≤ x ≤1. (Tina) (Hint: think integral)

7. (VA 1995) Let . Show that for every positive integer
n. Here [r] denotes the largest integer that is not larger than r. (David Rose) (Hint:
prove ≥ and ≤ both hold.)

8. Solve the equation z 8 + 4z6 - 10z4 + 4z2 + 1 = 0. (Lei) (Hint: divide it by z4, and
observe the symmetry)

9. (Putnam 2004-B1) Let be a polynomial with integer
coefficients. Suppose that r is a rational number such that P(r) = 0. Show that the
n numbers



are integers. (Davis Edmonson)

10. (Putnam 2003-B1) Do there exist polynomials a(x), b(x), c(y), d(y) such that

1 + xy + x2y2 = a(x)c(y) + b(x)d(y)

holds identically? (Richard)

More Problems:

1. If a and b are two solutions of x4 -x3 -1 = 0, then ab is a solution of x6 +x4 +x3 -
x2 - 1 = 0.

2. Suppose that a, b, c are distinctive integers. Prove

for any x ∈ R.

3. (VA 1997) Suppose that and . If and are roots of x4 - x3 +
ax2 - 8x - 8 = 0, find and a. (Do not assume that they are real numbers.)

4. (VA 1991) Let f(x) = x5 - 5x3 + 4x. In each part (i)–(iv), prove or disprove that
there exists a real number c for which f(x) - c = 0 has a root of multiplicity (i) one,
(ii) two, (iii) three, (iv) four.

5. (VA 1985) Let , where the coefficients are real. Prove that
p(x) = 0 has at least one root in the interval 0 ≤ x ≤1 if .

6. (VA 1989) Let a,b, c,d be distinct integers such that the equation (x - a)(x - b)(x -
c)(x-d)-9 = 0 has an integer root r. Show that 4r = a+b+c+d. (This is essentially
a problem from the 1947 Putnam examination.)

7. (VA 1988) Find positive real numbers a and b such that f(x) = ax - bx3 has four
extrema on [-1, 1], at each of which |f(x)| = 1.

8. (VA 1987) Let p(x) be given by and let |p(x)| ≤ |x|
on [-1, 1]. (a) Evaluate . (b) Prove that .

9. (VA 1990) Suppose that P(x) is a polynomial of degree 3 with integer coefficients and
that P
(1) = 0, P(2) = 0. Prove that at least one of its four coefficients is equal to or
less than -2.

10. (Putnam 2004-A4) Show that for any positive integer n, there is an integer N such
that the product can be expressed identically in the form

where the are rational numbers and each is one of the numbers -1, 0, 1.

11. (Putnam 2003-A4) Suppose that a, b, c, A,B,C are real numbers, a ≠ 0 and A ≠ 0,
such that

|ax2 + bx + c| ≤ |Ax2 + Bx + C|

for all real numbers x. Show that

|b2 - 4ac| ≤ |B2 - 4AC|.

12. (Putnam 2003-B1) Do there exist polynomials a(x), b(x), c(y), d(y) such that

1 + xy + x2y2 = a(x)c(y) + b(x)d(y)

holds identically?

13. (Putnam 2003-B4) Let



where a, b, c, d, e are integers, a ≠ 0. Show that if is a rational number and
, then is a rational number.

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