Let p, q, and r be the distinct roots of the polynomial x3−22x2+80x−67. It is given that there exist real numbers A, B, and C such that s3−22s2+80s−671=s−pA+s−qB+s−rCfor all s∈{p,q,r}. What is A1+B1+C1?
Pick one
Solution
Multiplying both sides by (s−p)(s−q)(s−r) yields 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 s, it must be true for all s (since a polynomial with infinitely many roots must in fact be the constant polynomial 0). This means we can plug in s=p to find that A1=(p−q)(p−r). Similarly, we can find B1=(q−p)(q−r) and C1=(r−p)(r−q). Summing them up, we get that A1+B1+C1=p2+q2+r2−pq−qr−pr We can express p2+q2+r2=(p+q+r)2−2(pq+qr+pr), and by Vieta's Formulas, we know that this expression is equal to 324. Vieta's also gives pq+qr+pr=80 (which we also used to find p2+q2+r2), so the answer is 324−80=(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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