3^a Questão:

(a) Seja p(x) em R[x], com gr(p(x)) = 4. Prove que se alpha = a + b i em C é raiz de p(x), então α = a - b i também é raiz de p(x).

Passo 1

Beleza, vamos lá! O enunciado nos diz que temos um polinômio com coeficientes reais e que o grau desse polinômio é 4 ( ). Além disso, sabemos que é uma raiz desse polinômio. Nosso objetivo é provar que a conjugada de , que é , também é uma raiz de .

Passo 2

Primeiro, vamos lembrar que se é uma raiz de , então substituindo na função temos:
Isso significa que quando a gente substitui por no polinômio, o resultado é zero.

Passo 3

Agora, vamos utilizar uma propriedade importante dos polinômios com coeficientes reais: se os coeficientes são reais e é uma raiz, então a sua conjugada também deve ser uma raiz. Mas por que isso acontece? Vamos entender melhor.

Passo 4

Como tem coeficientes reais, ao calcular o conjugado de , temos:
Por outro lado, podemos distribuir o conjugado dentro do polinômio:
Isso ocorre porque, ao ter coeficientes reais, o conjugado de cada termo do polinômio é igual ao termo com a conjugada de .

Passo 5

Então, juntando as duas igualdades que temos:
Isso mostra que também satisfaz a equação , ou seja, é uma raiz de .

Resposta

Portanto, se é uma raiz de , então sua conjugada também é uma raiz de .

Exercícios relacionados

Usando os teoremas acima e supondo que uma sequência a_n é tal que , calcule os seguintes limites:
(b)
<html><head></head><body><div><span class="math-inline ">
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<mjx-container class=“MathJax” jax=“SVG”><svg style=“vertical-align: -0.566ex;” xmlns=“http://www.w3.org/2000/svg” width=“25.454ex” height=“2.452ex” role=“img” focusable=“false” viewBox=“0 -833.9 11250.6 1083.9”><title>p(x) = x^2 - k x - 1 em Z[x]</title><g stroke=“currentColor” fill=“currentColor” stroke-width=“0” transform=“scale(1,-1)”><g data-mml-node=“math”><g data-mml-node=“mi”><path data-c=“1D45D” d=“M23 287Q24 290 25 295T30 317T40 348T55 381T75 411T101 433T134 442Q209 442 230 378L240 387Q302 442 358 442Q423 442 460 395T497 281Q497 173 421 82T249 -10Q227 -10 210 -4Q199 1 187 11T168 28L161 36Q160 35 139 -51T118 -138Q118 -144 126 -145T163 -148H188Q194 -155 194 -157T191 -175Q188 -187 185 -190T172 -194Q170 -194 161 -194T127 -193T65 -192Q-5 -192 -24 -194H-32Q-39 -187 -39 -183Q-37 -156 -26 -148H-6Q28 -147 33 -136Q36 -130 94 103T155 350Q156 355 156 364Q156 405 131 405Q109 405 94 377T71 316T59 280Q57 278 43 278H29Q23 284 23 287ZM178 102Q200 26 252 26Q282 26 310 49T356 107Q374 141 392 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244T261 278T284 308T305 340T320 369T333 401T340 431T343 464Q343 527 309 573T212 619Q179 619 154 602T119 569T109 550Q109 549 114 549Q132 549 151 535T170 489Q170 464 154 447T109 429Z”></path></g></g></g><g data-mml-node=“mo” transform=“translate(4417.3,0)”><path data-c=“2212” d=“M84 237T84 250T98 270H679Q694 262 694 250T679 230H98Q84 237 84 250Z”></path></g><g data-mml-node=“mi” transform=“translate(5417.6,0)”><path data-c=“1D458” d=“M121 647Q121 657 125 670T137 683Q138 683 209 688T282 694Q294 694 294 686Q294 679 244 477Q194 279 194 272Q213 282 223 291Q247 309 292 354T362 415Q402 442 438 442Q468 442 485 423T503 369Q503 344 496 327T477 302T456 291T438 288Q418 288 406 299T394 328Q394 353 410 369T442 390L458 393Q446 405 434 405H430Q398 402 367 380T294 316T228 255Q230 254 243 252T267 246T293 238T320 224T342 206T359 180T365 147Q365 130 360 106T354 66Q354 26 381 26Q429 26 459 145Q461 153 479 153H483Q499 153 499 144Q499 139 496 130Q455 -11 378 -11Q333 -11 305 15T277 90Q277 108 280 121T283 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transform=“translate(9455.6,0)”><g data-mml-node=“mi”><path data-c=“2124” d=“M39 -1Q29 9 29 12Q29 23 60 77T219 337L410 648H364Q261 648 210 628Q168 612 142 588T109 545T97 509T88 490Q85 489 80 489Q72 489 61 503L70 588Q72 607 75 628T79 662T81 675Q84 677 88 681Q90 683 341 683H592Q604 673 604 666Q604 662 412 348L221 37Q221 35 301 35Q406 35 446 48Q504 68 543 111T597 212Q602 239 617 239Q624 239 629 234T635 223Q635 215 621 113T604 8L597 1Q595 -1 317 -1H39ZM148 637L166 648H112V632Q111 629 110 622T108 612Q108 608 110 608T116 612T129 623T148 637ZM552 646Q552 648 504 648Q452 648 450 643Q448 639 266 343T77 37Q77 35 128 35H179L366 339L552 646ZM572 35Q581 89 581 97L561 77Q542 59 526 48L508 37L539 35H572Z”></path></g></g><g data-mml-node=“mo” transform=“translate(10122.6,0)”><path data-c=“5B” d=“M118 -250V750H255V710H158V-210H255V-250H118Z”></path></g><g data-mml-node=“mi” transform=“translate(10400.6,0)”><path data-c=“1D465” d=“M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z”></path></g><g data-mml-node=“mo” transform=“translate(10972.6,0)”><path data-c=“5D” d=“M22 710V750H159V-250H22V-210H119V710H22Z”></path></g></g></g></svg></mjx-container></span> possui uma raiz <span class="math-inline ">
<mjx-container class=“MathJax” jax=“SVG”><svg style=“vertical-align: -0.09ex;” xmlns=“http://www.w3.org/2000/svg” width=“5.723ex” height=“1.636ex” role=“img” focusable=“false” viewBox=“0 -683 2529.6 723”><title>alpha in Z</title><g stroke=“currentColor” fill=“currentColor” stroke-width=“0” transform=“scale(1,-1)”><g data-mml-node=“math”><g data-mml-node=“mi”><path data-c=“1D6FC” d=“M34 156Q34 270 120 356T309 442Q379 442 421 402T478 304Q484 275 485 237V208Q534 282 560 374Q564 388 566 390T582 393Q603 393 603 385Q603 376 594 346T558 261T497 161L486 147L487 123Q489 67 495 47T514 26Q528 28 540 37T557 60Q559 67 562 68T577 70Q597 70 597 62Q597 56 591 43Q579 19 556 5T512 -10H505Q438 -10 414 62L411 69L400 61Q390 53 370 41T325 18T267 -2T203 -11Q124 -11 79 39T34 156ZM208 26Q257 26 306 47T379 90L403 112Q401 255 396 290Q382 405 304 405Q235 405 183 332Q156 292 139 224T121 120Q121 71 146 49T208 26Z”></path></g><g data-mml-node=“mo” transform=“translate(917.8,0)”><path data-c=“2208” d=“M84 250Q84 372 166 450T360 539Q361 539 377 539T419 540T469 540H568Q583 532 583 520Q583 511 570 501L466 500Q355 499 329 494Q280 482 242 458T183 409T147 354T129 306T124 272V270H568Q583 262 583 250T568 230H124V228Q124 207 134 177T167 112T231 48T328 7Q355 1 466 0H570Q583 -10 583 -20Q583 -32 568 -40H471Q464 -40 446 -40T417 -41Q262 -41 172 45Q84 127 84 250Z”></path></g><g data-mml-node=“TeXAtom” data-mjx-texclass=“ORD” transform=“translate(1862.6,0)”><g data-mml-node=“mi”><path data-c=“2124” d=“M39 -1Q29 9 29 12Q29 23 60 77T219 337L410 648H364Q261 648 210 628Q168 612 142 588T109 545T97 509T88 490Q85 489 80 489Q72 489 61 503L70 588Q72 607 75 628T79 662T81 675Q84 677 88 681Q90 683 341 683H592Q604 673 604 666Q604 662 412 348L221 37Q221 35 301 35Q406 35 446 48Q504 68 543 111T597 212Q602 239 617 239Q624 239 629 234T635 223Q635 215 621 113T604 8L597 1Q595 -1 317 -1H39ZM148 637L166 648H112V632Q111 629 110 622T108 612Q108 608 110 608T116 612T129 623T148 637ZM552 646Q552 648 504 648Q452 648 450 643Q448 639 266 343T77 37Q77 35 128 35H179L366 339L552 646ZM572 35Q581 89 581 97L561 77Q542 59 526 48L508 37L539 35H572Z”></path></g></g></g></g></svg></mjx-container></span>. Determine <span class="math-inline " data-overflow=“visible”>
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<mjx-container class=“MathJax” jax=“SVG”><svg style=“vertical-align: -0.566ex;” xmlns=“http://www.w3.org/2000/svg” width=“4.192ex” height=“2.262ex” role=“img” focusable=“false” viewBox=“0 -750 1853 1000”><title>p(x)</title><g stroke=“currentColor” fill=“currentColor” stroke-width=“0” transform=“scale(1,-1)”><g data-mml-node=“math”><g data-mml-node=“mi”><path data-c=“1D45D” d=“M23 287Q24 290 25 295T30 317T40 348T55 381T75 411T101 433T134 442Q209 442 230 378L240 387Q302 442 358 442Q423 442 460 395T497 281Q497 173 421 82T249 -10Q227 -10 210 -4Q199 1 187 11T168 28L161 36Q160 35 139 -51T118 -138Q118 -144 126 -145T163 -148H188Q194 -155 194 -157T191 -175Q188 -187 185 -190T172 -194Q170 -194 161 -194T127 -193T65 -192Q-5 -192 -24 -194H-32Q-39 -187 -39 -183Q-37 -156 -26 -148H-6Q28 -147 33 -136Q36 -130 94 103T155 350Q156 355 156 364Q156 405 131 405Q109 405 94 377T71 316T59 280Q57 278 43 278H29Q23 284 23 287ZM178 102Q200 26 252 26Q282 26 310 49T356 107Q374 141 392 215T411 325V331Q411 405 350 405Q339 405 328 402T306 393T286 380T269 365T254 350T243 336T235 326L232 322Q232 321 229 308T218 264T204 212Q178 106 178 102Z”></path></g><g data-mml-node=“mo” transform=“translate(503,0)”><path data-c=“28” d=“M94 250Q94 319 104 381T127 488T164 576T202 643T244 695T277 729T302 750H315H319Q333 750 333 741Q333 738 316 720T275 667T226 581T184 443T167 250T184 58T225 -81T274 -167T316 -220T333 -241Q333 -250 318 -250H315H302L274 -226Q180 -141 137 -14T94 250Z”></path></g><g data-mml-node=“mi” transform=“translate(892,0)”><path data-c=“1D465” d=“M52 289Q59 331 106 386T222 442Q257 442 286 424T329 379Q371 442 430 442Q467 442 494 420T522 361Q522 332 508 314T481 292T458 288Q439 288 427 299T415 328Q415 374 465 391Q454 404 425 404Q412 404 406 402Q368 386 350 336Q290 115 290 78Q290 50 306 38T341 26Q378 26 414 59T463 140Q466 150 469 151T485 153H489Q504 153 504 145Q504 144 502 134Q486 77 440 33T333 -11Q263 -11 227 52Q186 -10 133 -10H127Q78 -10 57 16T35 71Q35 103 54 123T99 143Q142 143 142 101Q142 81 130 66T107 46T94 41L91 40Q91 39 97 36T113 29T132 26Q168 26 194 71Q203 87 217 139T245 247T261 313Q266 340 266 352Q266 380 251 392T217 404Q177 404 142 372T93 290Q91 281 88 280T72 278H58Q52 284 52 289Z”></path></g><g data-mml-node=“mo” transform=“translate(1464,0)”><path data-c=“29” d=“M60 749L64 750Q69 750 74 750H86L114 726Q208 641 251 514T294 250Q294 182 284 119T261 12T224 -76T186 -143T145 -194T113 -227T90 -246Q87 -249 86 -250H74Q66 -250 63 -250T58 -247T55 -238Q56 -237 66 -225Q221 -64 221 250T66 725Q56 737 55 738Q55 746 60 749Z”></path></g></g></g></svg></mjx-container></span>.</div>
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Dados e , determine:
(a) ;
(b) .
Usando os teoremas acima e supondo que uma sequência a_n é tal que , calcule os seguintes limites:
(f)