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2019 AMC 10A Problem 24

Problem 24 of 25HarderAlgebra

Let p,p, q,q, and rr be the distinct roots of the polynomial x322x2+80x67.x^3 - 22x^2 + 80x - 67. There exist real numbers A,A, B,B, and CC such that 1s322s2+80s67=Asp+Bsq+Csr \begin{gathered} \frac{1}{s^3-22s^2+80s-67}\\ =\frac{A}{s-p}+\frac{B}{s-q}\\ \quad+\frac{C}{s-r} \end{gathered} for all real numbers ss with s{p,q,r}.s\notin\{p,q,r\}. What is 1A+1B+1C?\dfrac1A+\dfrac1B+\dfrac1C?

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Solution

Multiplying the identity by (sp)(sq)(sr)(s-p)(s-q)(s-r) gives 1=A(sq)(sr)+B(sp)(sr)+C(sp)(sq). \begin{aligned} 1={}&A(s-q)(s-r)\\ &+B(s-p)(s-r)\\ &+C(s-p)(s-q). \end{aligned} Setting s=p,q,rs=p,q,r, in turn, yields 1A=(pq)(pr),1B=(qp)(qr),1C=(rp)(rq). \begin{aligned} \frac1A&=(p-q)(p-r),\\ \frac1B&=(q-p)(q-r),\\ \frac1C&=(r-p)(r-q). \end{aligned} Adding and expanding gives 1A+1B+1C=p2+q2+r2pqprqr. \begin{aligned} \frac1A+\frac1B+\frac1C &=p^2+q^2+r^2\\ &\quad-pq-pr-qr. \end{aligned} By Vieta’s formulas, p+q+r=22p+q+r=22 and pq+pr+qr=80pq+pr+qr=80, so p2+q2+r2=2222(80)=324. \begin{aligned} p^2+q^2+r^2&=22^2-2(80)\\ &=324. \end{aligned} Therefore the requested value is 32480=244.324-80=244. Thus, B is the correct answer.

More practice

Concepts: partial fractions · Vieta’s Formulas

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.