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Algebra Difficulty 3.8 AMC 10/12 Find the answer

Let pp, qq, and rr be the distinct roots of the polynomial x322x2+80x67x^3 - 22x^2 + 80x - 67. It is given that there exist real numbers AA, BB, and CC such that 1s322s2+80s67=Asp+Bsq+Csr\dfrac{1}{s^3 - 22s^2 + 80s - 67} = \dfrac{A}{s-p} + \dfrac{B}{s-q} + \frac{C}{s-r}for all s∉{p,q,r}s\not\in\{p,q,r\}. What is 1A+1B+1C\tfrac1A+\tfrac1B+\tfrac1C?

Pick one

Solution

Multiplying both sides by (sp)(sq)(sr)(s-p)(s-q)(s-r) yields
1=A(sq)(sr)+B(sp)(sr)+C(sp)(sq)1 = A(s-q)(s-r) + B(s-p)(s-r) + C(s-p)(s-q)
As this is a polynomial identity, and it is true for infinitely many ss, it must be true for all ss (since a polynomial with infinitely many roots must in fact be the constant polynomial 00). This means we can plug in s=ps = p to find that 1A=(pq)(pr)\frac1A = (p-q)(p-r). Similarly, we can find 1B=(qp)(qr)\frac1B = (q-p)(q-r) and 1C=(rp)(rq)\frac1C = (r-p)(r-q). Summing them up, we get that 1A+1B+1C=p2+q2+r2pqqrpr\frac1A + \frac1B + \frac1C = p^2 + q^2 + r^2 - pq - qr - pr
We can express p2+q2+r2=(p+q+r)22(pq+qr+pr)p^2 + q^2 + r^2 = (p+q+r)^2 - 2(pq + qr + pr), and by Vieta's Formulas, we know that this expression is equal to 324324. Vieta's also gives pq+qr+pr=80pq + qr + pr = 80 (which we also used to find p2+q2+r2p^2+q^2+r^2), so the answer is 32480=(B) 244324 -80 = \boxed{\textbf{(B) } 244}.
Note: this process of substituting in the 'forbidden' values in the original identity is a standard technique for partial fraction decomposition, as taught in calculus classes.
-very small latex edit from countmath1 :)
Minor rephrasing for correctness and clarity ~ Technodoggo

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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.