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Algebra Difficulty 3.8 AMC 10/12 Find the answer

Consider polynomials P(x)P(x) of degree at most 33, each of whose coefficients is an element of {0,1,2,3,4,5,6,7,8,9}\{0, 1, 2, 3, 4, 5, 6, 7, 8, 9\}. How many such polynomials satisfy P(1)=9P(-1) = -9?

Pick one

Solutions — 2

Solution 1

Suppose that P(x)=ax3+bx2+cx+d.P(x)=ax^3+bx^2+cx+d. This problem is equivalent to counting the ordered quadruples (a,b,c,d),(a,b,c,d), where all of a,b,c,a,b,c, and dd are integers from 00 through 99 such that P(1)=a+bc+d=9.P(-1)=-a+b-c+d=-9. Let a=9aa'=9-a and c=9c.c'=9-c. Note that both of aa' and cc' are integers from 00 through 9.9. Moreover, the ordered quadruples (a,b,c,d)(a,b,c,d) and the ordered quadruples (a,b,c,d)(a',b,c',d) have one-to-one correspondence.
We rewrite the given equation as (9a)+b+(9c)+d=9,(9-a)+b+(9-c)+d=9, or a+b+c+d=9.a'+b+c'+d=9. By the stars and bars argument, there are (9+4141)=(D) 220\binom{9+4-1}{4-1}=\boxed{\textbf{(D) } 220} ordered quadruples (a,b,c,d).(a',b,c',d).
~pieater314159 ~MRENTHUSIASM

Solution 2

1. Let P(x)=ax3+bx2+cx+d P(x) = ax^3 + bx^2 + cx + d . We are given that P(1)=9 P(-1) = -9 .
2. Substituting x=1 x = -1 into P(x) P(x) , we get:
P(1)=a(1)3+b(1)2+c(1)+d=a+bc+d P(-1) = a(-1)^3 + b(-1)^2 + c(-1) + d = -a + b - c + d
3. We need a+bc+d=9 -a + b - c + d = -9 . Rearranging, we get:
b+d=a+c+9 b + d = a + c + 9
4. Let s=a+c+9 s = a + c + 9 . Then b+d=s b + d = s . The possible values of s s range from 9 to 18 (since a,b,c,d a, b, c, d are digits from 0 to 9).
5. For each fixed s s , we need to count the number of valid pairs (a,c)(a, c) and (b,d)(b, d) such that:
a+c=s9andb+d=s a + c = s - 9 \quad \text{and} \quad b + d = s
6. The number of pairs (a,c)(a, c) such that a+c=s9 a + c = s - 9 is s8 s - 8 (since a a and c c are digits from 0 to 9).
7. The number of pairs (b,d)(b, d) such that b+d=s b + d = s is 19s 19 - s (since b b and d d are digits from 0 to 9).
8. Therefore, for each s s , the number of valid polynomials is:
(s8)(19s) (s - 8)(19 - s)
9. We need to sum this product over all possible values of s s from 9 to 18:
s=918(s8)(19s) \sum_{s=9}^{18} (s - 8)(19 - s)
10. Simplifying the sum:
s=918(s8)(19s)=s=09(s+1)(10s) \sum_{s=9}^{18} (s - 8)(19 - s) = \sum_{s=0}^{9} (s + 1)(10 - s)
11. Calculating each term:
(0+1)(100)=110=10(1+1)(101)=29=18(2+1)(102)=38=24(3+1)(103)=47=28(4+1)(104)=56=30(5+1)(105)=65=30(6+1)(106)=74=28(7+1)(107)=83=24(8+1)(108)=92=18(9+1)(109)=101=10 \begin{aligned} & (0 + 1)(10 - 0) = 1 \cdot 10 = 10 \\ & (1 + 1)(10 - 1) = 2 \cdot 9 = 18 \\ & (2 + 1)(10 - 2) = 3 \cdot 8 = 24 \\ & (3 + 1)(10 - 3) = 4 \cdot 7 = 28 \\ & (4 + 1)(10 - 4) = 5 \cdot 6 = 30 \\ & (5 + 1)(10 - 5) = 6 \cdot 5 = 30 \\ & (6 + 1)(10 - 6) = 7 \cdot 4 = 28 \\ & (7 + 1)(10 - 7) = 8 \cdot 3 = 24 \\ & (8 + 1)(10 - 8) = 9 \cdot 2 = 18 \\ & (9 + 1)(10 - 9) = 10 \cdot 1 = 10 \\ \end{aligned}
12. Summing these values:
10+18+24+28+30+30+28+24+18+10=220 10 + 18 + 24 + 28 + 30 + 30 + 28 + 24 + 18 + 10 = 220

The final answer is 220\boxed{220}

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