The equation x10+(13x−1)10=0has 10 complex roots r1,r1,r2,r2,r3,r3,r4,r4,r5,r5, where the bar denotes complex conjugation. Find the value of r1r11+r2r21+r3r31+r4r41+r5r51.
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Let p(x)=x10+(13x−1)10. If r is a root of p(x), then r10+(13x−1)10=0. Then (13r−1)10=−r10, so −1=(r13r−1)10=(r1−13)10.Then r1−13 has magnitude 1, so (r1−13)(r1−13)=1,so (r11−13)(r11−13)+⋯+(r51−13)(r51−13)=5.Expanding, we get r1r11+⋯+r5r51−13(r11+r11+⋯+r51+r51)+5⋅169=5.We see that r11,r11,…,r51,r51 are the solutions to (x1)10+(x13−1)10=0,or 1+(13−x)10=0. The first few terms in the expansion as x10−130x9+⋯=0,so by Vieta's formulas, r11+r11+⋯+r51+r51=130.Hence, r1r11+⋯+r5r51=13⋅130−5⋅169+5=850.