2019 AMC 10A Problem 24

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Concepts:partial fractionsVieta’s Formulas

Difficulty rating: 2390

24.

Let p,p, q,q, and rr be the distinct roots of the polynomial x322x2+80x67.x^3 - 22x^2 + 80x - 67. It is given that there exist real numbers A,A, B,B, and CC such that 1s322s2+80s67=\dfrac{1}{s^3 - 22s^2 + 80s - 67} =Asp+Bsq+Csr \dfrac{A}{s-p} + \dfrac{B}{s-q} + \dfrac{C}{s-r}for all s∉{p,q,r}.s\not\in\{p,q,r\}. What is 1A+1B+1C?\dfrac1A+\dfrac1B+\dfrac1C?

243243

244244

245245

246246

247247

Solution:

We can multiply each side by (sp)(sq)(sr) (s - p)(s - q)(s - r) to get A(sq)(sr)+B(sp)(sr) A(s - q)(s - r) + B(s - p)(s - r)+C(sp)(sq)=1. + C(s - p)(s - q) = 1.

We can expand to get s2(A+B+C)s s^2(A + B + C) - s \cdot (Aq+Ar+Bp+Br+Cp+Cq) (Aq + Ar + Bp + Br + Cp + Cq) +(Aqr+Bpr+Cpq1)=0. + (Aqr + Bpr + Cpq - 1) = 0.

Note that the coefficients of ss and s2s^2 must both be 0.0.

From A+B+C=0,(1) A + B + C = 0, \tag*{(1)} we get A=(B+C), A = -(B + C),B=(A+C), B = -(A + C), and C=(A+B). C = -(A + B).

Plugging this into the coefficient of s,s, we get Ap+Bq+Cr=0. Ap + Bq + Cr = 0.

Subtracting (1)r(1) \cdot r from this equation, we get A(pr)+B(qr)=0(2) A(p - r) + B(q - r) = 0 (2) We also know that Aqr+Bpr+Cpq=1. Aqr + Bpr + Cpq = 1. Subtracting (1)pq(1) \cdot pq from this equation, we get Aq(rp)+Bp(rq)=1. Aq(r - p) + Bp(r - q) = 1.

Adding (2)p(2) \cdot p to this equation, we get A(rp)(qp)=1, A(r - p)(q - p) = 1, which gets us A=1(rp)(qp). A = \dfrac{1}{(r - p)(q - p)}.

Similarly, we get B=1(rq)(pq) B = \dfrac{1}{(r - q)(p - q)} and C=1(qr)(pr). C = \dfrac{1}{(q - r)(p - r)}.

This gives us 1A+1B+1C= \dfrac{1}{A} + \dfrac{1}{B} + \dfrac{1}{C} = p2+q2+r2pqqrpr. p^2 + q^2 + r^2 - pq - qr - pr.

Using Vieta's formulas, we get p2+q2+r2=(p+q+r)2 p^2 + q^2 + r^2 = (p + q + r)^2 - 2(pq+qr+pr)=324. 2(pq + qr + pr) = 324. Finally, since pq+qr+pr=80, pq + qr + pr = 80, we get our desired value of 32480=244.324 - 80 = 244.

Thus, B is the correct answer.

Problem 24 in Other Years