[중3-1] 인수분해 연산 문제 (1000제)
📌 이전 학년 내용 정리: 분배법칙과 인수
인수분해는 하나의 다항식을 두 개 이상의 단항식이나 다항식의 곱으로 나타내는 것으로, 전개의 역과정입니다. 인수분해의 가장 기본 원리는 여러 항에 공통으로 들어있는 인수를 묶어내는 분배법칙의 역입니다.
- 전개 (분배법칙): $m(a+b) \rightarrow ma + mb$
- 인수분해 (공통인수로 묶기): $ma + mb \rightarrow m(a+b)$
이때 곱을 이루는 각각의 식 $m$과 $(a+b)$를 다항식 $ma+mb$의 인수라고 합니다.
I. 공통인수를 이용한 인수분해 (1-100)
1. $2a+2b$
2. $5x-5y$
3. $ax-ay$
4. $mxy+myz$
5. $3x^2+6x$
6. $4a^2-8ab$
7. $x^2y+xy^2$
8. $6a^2b-9ab$
9. $10x^2-5x$
10. $-2x-4y$
11. $a^2b+a$
12. $xy^2-y$
13. $ax+bx+cx$
14. $2xy-4yz+6zx$
15. $a^3-a^2+a$
16. $8x^2y-12xy^2$
17. $15a^2b+25ab^2$
18. $x(a+b)+y(a+b)$
19. $a(x-2)+b(x-2)$
20. $m(a-b)-n(a-b)$
21. $x(2a+b)+y(2a+b)$
22. $(x+1)a-(x+1)b$
23. $a(x-y)-b(y-x)$
24. $x(a-b)+b-a$
25. $2y(x-1)-(1-x)$
26. $3x^2y-6xy^2+9xy$
27. $a^2b^3+a^3b^2$
28. $14x^2-21xy$
29. $18a^2b-24b^2$
30. $(a+b)^2+2(a+b)$
31. $4x^2(y-1)+6x(y-1)$
32. $xy-x+y-1$
33. $ab+a+b+1$
34. $ax-bx+ay-by$
35. $a^2+ab+ac+bc$
36. $xy-x-y+1$
37. $a^3+a^2+a+1$
38. $x^2-xy-2x+2y$
39. $ab-3a-2b+6$
40. $4ab-2a+6b-3$
41. $x^2+x^3$
42. $y^4-y^2$
43. $6m^2-2m$
44. $ab+ac$
45. $7xy-14y$
46. $9x^2+3x$
47. $a^2b+ab^2$
48. $4x^2y-8xy$
49. $-5a^2b-10ab$
50. $x(y+z)+2(y+z)$
51. $a(b-3)-2(b-3)$
52. $p(q+r)-q-r$
53. $x^2-ax-bx+ab$
54. $y^2+cy-dy-cd$
55. $2ax+4ay-bx-2by$
56. $z^3-z^2+z-1$
57. $m^2-mn-m+n$
58. $ac+a-c-1$
59. $x^2y+x^2-y-1$
60. $a-2b+2a^2-4ab$
61. $2x^2+2x$
62. $a^3-a^2$
63. $6y^2+3y$
64. $8b-4b^2$
65. $xy-x^2y$
66. $ab^2-a^2b$
67. $5m^2n+10mn$
68. $9x^2y-12x^2y^2$
69. $-7a^3-14a$
70. $x(a-b)-2(a-b)$
71. $a(x+3)+b(x+3)$
72. $3(y-z)-x(y-z)$
73. $2m(n-1)+3(1-n)$
74. $a(x-5)-5+x$
75. $3(p-q)-q+p$
76. $(x-y)^2-3(y-x)$
77. $4a^2b^3-6a^3b^2+8a^2b^2$
78. $15x^3y^2+20x^2y^3-5x^2y^2$
79. $x(a+b+c)+y(a+b+c)$
80. $(a+1)^2+(a+1)$
81. $4x^2+8xy$
82. $9ab-6b^2$
83. $a^2b-ab+ab^2$
84. $x^3-2x^2+x$
85. $-3a^2+9a$
86. $m(x-y)-n(y-x)$
87. $2a(x-1)+3(1-x)$
88. $x(a-b)+y(b-a)$
89. $12x^3y^2-16x^2y^3$
90. $2(x+y)^2-x-y$
91. $xy-x+2-2y$
92. $a^2+3a-ab-3b$
93. $2x+y-2xy-x^2$
94. $1-a+b-ab$
95. $m^3-1-m+m^2$
96. $yz+y-z-1$
97. $2ab-4a-3b+6$
98. $x^3+2x^2-9x-18$
99. $a(a-b)-c(b-a)$
100. $3x^2(y-z)-(z-y)$
II. 인수분해 공식 (완전제곱식 & 합차) (101-400)
Part 1: $a^2 \pm 2ab + b^2 = (a \pm b)^2$ (101-250)
101. $x^2 + 4x + 4$
102. $a^2 + 12a + 36$
103. $y^2 – 8y + 16$
104. $b^2 – 14b + 49$
105. $x^2 + 20x + 100$
106. $x^2 – 22x + 121$
107. $4a^2 + 12a + 9$
108. $9x^2 – 30x + 25$
109. $16y^2 + 8y + 1$
110. $25 – 40a + 16a^2$
111. $x^2 + 2xy + y^2$
112. $a^2 – 6ab + 9b^2$
113. $4x^2 + 20xy + 25y^2$
114. $9a^2 – 24ab + 16b^2$
115. $m^2 + m + \frac{1}{4}$
116. $x^2 – \frac{2}{3}x + \frac{1}{9}$
117. $\frac{1}{4}a^2 + a + 1$
118. $x^2 + \frac{4}{3}x + \frac{4}{9}$
119. $3x^2 + 6x + 3$
120. $2a^2 – 20a + 50$
121. $5y^2 – 10y + 5$
122. $-x^2 – 12x – 36$
123. $-2a^2 + 8ab – 8b^2$
124. $ax^2 + 2axy + ay^2$
125. $4x^3+4x^2+x$
126. $a^2 + 18a + 81$
127. $x^2 – 2x + 1$
128. $b^2 + 24b + 144$
129. $y^2 – 26y + 169$
130. $1 + 4a + 4a^2$
131. $9 – 12x + 4x^2$
132. $36a^2+12a+1$
133. $49x^2-28x+4$
134. $x^2-18xy+81y^2$
135. $25a^2+40ab+16b^2$
136. $49m^2-70mn+25n^2$
137. $a^2-ab+\frac{1}{4}b^2$
138. $\frac{1}{9}x^2+\frac{2}{3}xy+y^2$
139. $x^2y^2-2xy+1$
140. $10x^2+20x+10$
141. $4a^2-16a+16$
142. $-3y^2+42y-147$
143. $\frac{1}{2}x^2+4x+8$
144. $x^2+2x+1$
145. $a^2-4a+4$
146. $y^2+10y+25$
147. $m^2-12m+36$
148. $x^2+14x+49$
149. $9a^2+6a+1$
150. $16b^2-8b+1$
151. $25x^2+20x+4$
152. $4-12y+9y^2$
153. $1+2a^2+a^4$
154. $x^2+16xy+64y^2$
155. $4a^2-12ab+9b^2$
156. $m^2+2mn+n^2$
157. $x^2-\frac{1}{3}x+\frac{1}{36}$
158. $4a^2+a+\frac{1}{16}$
159. $3x^2+42x+147$
160. $a^2x^2-2ax+1$
161. $x^2 + \square + 16$
162. $y^2 + \square + 25$
163. $a^2 – \square + 64$
164. $b^2 – \square + 100$
165. $4x^2 + \square + 1$
166. $9x^2 – \square + 4$
167. $25y^2 + \square + 9$
168. $49a^2 – \square + 16b^2$
169. $x^2 + 4x + \square$
170. $a^2 + 8a + \square$
171. $y^2 – 12y + \square$
172. $b^2 – 18b + \square$
173. $x^2 + xy + \square$
174. $a^2 – 6ab + \square$
175. $4x^2 + 20x + \square$
176. $9y^2 – 42y + \square$
177. $16x^2 + 8xy + \square$
178. $25a^2 – 30ab + \square$
179. $x^2+x+\square$
180. $a^2-3a+\square$
181. $a^2+\square a+144$
182. $x^2+\square x+121$
183. $y^2-\square y+169$
184. $b^2-\square b+225$
185. $x^2+\square x+\frac{1}{4}$
186. $a^2-\square a+\frac{1}{25}$
187. $4x^2+\square x+9$
188. $9y^2-\square y+16$
189. $25x^2+\square xy+y^2$
190. $100a^2-\square ab+49b^2$
191. $ax^2+12x+9$이 완전제곱식이 될 때, $a$의 값은?
192. $x^2+ax+1$이 완전제곱식이 될 때, $a$의 값을 모두 구하시오.
193. $x^2-18x+c$가 완전제곱식이 될 때, $c$의 값은?
194. $x^2+11x+k$가 완전제곱식이 될 때, $k$의 값은?
195. $25x^2+kx+4$가 완전제곱식이 될 때, 양수 $k$의 값은?
196. $x^2-px+\frac{1}{16}$이 완전제곱식이 될 때, 양수 $p$의 값은?
197. $5x^2-20x+a$가 완전제곱식일 때, $a$의 값은?
198. $3x^2+bx+3$이 완전제곱식일 때, 양수 $b$의 값은?
199. $x^2+(2k-4)x+49$가 완전제곱식일 때, 양수 $k$의 값은?
200. $x^2+10xy+k y^2$가 완전제곱식일 때, $k$의 값은?
201. $x^2+2x+1$
202. $a^2-2a+1$
203. $y^2+4y+4$
204. $b^2-6b+9$
205. $m^2+8m+16$
206. $n^2-10n+25$
207. $x^2+12x+36$
208. $y^2-14y+49$
209. $4a^2+4a+1$
210. $9b^2-6b+1$
211. $25x^2+10x+1$
212. $16y^2-8y+1$
213. $1+6a+9a^2$
214. $4-4b+b^2$
215. $49x^2+14x+1$
216. $64y^2-16y+1$
217. $x^2+2xy+y^2$
218. $a^2-2ab+b^2$
219. $m^2+4mn+4n^2$
220. $9x^2-6xy+y^2$
221. $16a^2+24ab+9b^2$
222. $25p^2-30pq+9q^2$
223. $x^2-x+\frac{1}{4}$
224. $y^2+y+\frac{1}{4}$
225. $a^2+\frac{2}{3}a+\frac{1}{9}$
226. $b^2-\frac{1}{2}b+\frac{1}{16}$
227. $\frac{1}{9}x^2+\frac{2}{3}x+1$
228. $4y^2-2y+\frac{1}{4}$
229. $5x^2+10x+5$
230. $2a^2-12a+18$
231. $3b^2+18b+27$
232. $ax^2-2ax+a$
233. $-y^2+2y-1$
234. $-3x^2-6x-3$
235. $x^3+2x^2+x$
236. $2a^2b+4ab+2b$
237. $x^2-24x+\square$
238. $a^2+\square a+1$
239. $4y^2+\square+25$
240. $9x^2-3xy+\square$
241. $x^2+\frac{1}{3}x+\square$
242. $\square x^2+20x+25$
243. $\square a^2-12ab+4b^2$
244. $49x^2+\square+1$
245. $x^2-28x+\square$
246. $9x^2-60x+\square$
247. $x^2+\square x+196$
248. $a^2+\square ab+121b^2$
249. $x^2+(k+3)x+16$이 완전제곱식이 되기 위한 양수 $k$의 값은?
250. $(x+2)(x-4)+k$가 완전제곱식이 되도록 하는 상수 $k$의 값을 구하시오.
Part 2: $a^2 – b^2 = (a+b)(a-b)$ (251-400)
251. $x^2 – 4$
252. $a^2 – 25$
253. $y^2 – 81$
254. $b^2 – 144$
255. $1 – x^2$
256. $16 – a^2$
257. $49 – y^2$
258. $100 – b^2$
259. $4x^2 – 9$
260. $9a^2 – 1$
261. $16y^2 – 25$
262. $36b^2 – 1$
263. $25 – 4x^2$
264. $81 – 16a^2$
265. $x^2 – y^2$
266. $a^2 – 9b^2$
267. $4x^2 – y^2$
268. $25m^2 – 49n^2$
269. $a^2b^2 – 1$
270. $x^2y^2 – z^2$
271. $4x^2y^2 – 9$
272. $16a^2 – 25b^2c^2$
273. $2a^2 – 18$
274. $3x^2 – 27$
275. $5y^2 – 80$
276. $a^3 – 4a$
277. $x^3y – xy^3$
278. $8x^2 – 2y^2$
279. $-x^2 + y^2$
280. $-9a^2 + 4b^2$
281. $y^2 – \frac{1}{4}$
282. $a^2 – \frac{4}{25}$
283. $\frac{1}{9}x^2 – y^2$
284. $\frac{a^2}{49} – \frac{b^2}{64}$
285. $x^4 – 16$
286. $81 – a^4$
287. $m^4 – n^4$
288. $16x^4 – 1$
289. $(x+1)^2 – 4$
290. $(a-3)^2 – b^2$
291. $25 – (x+y)^2$
292. $9a^2 – (b-1)^2$
293. $(x+y)^2 – (x-y)^2$
294. $(2a+b)^2 – (a+b)^2$
295. $(3x-1)^2 – (x+2)^2$
296. $50x^2-2$
297. $45-5y^2$
298. $x^5-x$
299. $-18a^2+8b^2$
300. $a^8-1$
301. $x^2-16$
302. $a^2-121$
303. $y^2-1$
304. $4x^2-25$
305. $9a^2-100$
306. $16-49y^2$
307. $1-64b^2$
308. $x^2-25y^2$
309. $4a^2-81b^2$
310. $36m^2-n^2$
311. $p^2q^2-4$
312. $6x^2-6$
313. $2a^2-32b^2$
314. $27x^2-12y^2$
315. $b-ab^3$
316. $-2x^2+50$
317. $-a^2+144$
318. $x^2-\frac{1}{25}$
319. $\frac{1}{100}a^2-b^2$
320. $x^4-y^4$
321. $81a^4-b^4$
322. $(x+y)^2-1$
323. $a^2-(b+c)^2$
324. $(a-1)^2-(b-1)^2$
325. $(x+y)^2-(z+w)^2$
326. $a^2-1$
327. $y^2-36$
328. $x^2-225$
329. $9-a^2$
330. $4x^2-49$
331. $64-25b^2$
332. $16m^2-n^2$
333. $3a^2-75$
334. $y^3-16y$
335. $-4x^2+100$
336. $a^2-\frac{b^2}{4}$
337. $x^4-256$
338. $(x-2y)^2-9$
339. $1-(a+b)^2$
340. $(2x+3y)^2-(x-y)^2$
341. $x^2-36y^2$
342. $a^2b^2-c^2$
343. $5x^2-45y^2$
344. $7a-7a^3$
345. $x^8-1$
346. $(3a-4)^2-b^2$
347. $(x-2)^2-(y+2)^2$
348. $x^2-y^2$
349. $100x^2-121y^2$
350. $ab^2-a^3$
351. $25x^2 – 1$
352. $81 – y^2$
353. $x^2 – 169$
354. $64a^2 – 1$
355. $4 – 9b^2$
356. $a^2 – 100b^2$
357. $49x^2 – 144y^2$
358. $6a^2 – 24$
359. $2y – 2y^3$
360. $-12x^2 + 75$
361. $x^2 – \frac{y^2}{100}$
362. $16x^4 – y^4$
363. $(a+2b)^2 – c^2$
364. $16 – (x-y)^2$
365. $(a+2)^2 – (b+3)^2$
366. $a^2b^2 – 9$
367. $x^2y^2 – z^2a^2$
368. $2x^2 – 50$
369. $y^3 – 49y$
370. $3-3a^2$
371. $16-x^4$
372. $(2a+1)^2-1$
373. $4x^2-(y-z)^2$
374. $100x^2-1$
375. $a^2-25b^2$
376. $3x^2-12$
377. $x^3-64x$
378. $x^2-\frac{1}{y^2}$
379. $a^4-b^2$
380. $(x+3)^2-y^2$
381. $49-(a+b)^2$
382. $(2x+y)^2-9$
383. $1-(a-b)^2$
384. $(a+b)^2-(c-d)^2$
385. $-4x^2+9y^2$
386. $2y^2-50$
387. $x^4-x^2$
388. $\frac{x^2}{4}-\frac{y^2}{9}$
389. $a^2-400$
390. $121-x^2$
391. $x^2-196$
392. $4a^2-121$
393. $169-9y^2$
394. $49a^2-b^2$
395. $x^2y^2-100$
396. $7x^2-28$
397. $2a^3-2a$
398. $x^8-y^8$
399. $(2x-1)^2-(x+1)^2$
400. $(x-y)^2-4z^2$
V. 인수분해 공식 (이차식) (401-700)
Part 1: $x^2 + bx + c$ (401-550)
401. $x^2+4x+3$
402. $a^2+6a+8$
403. $y^2+8y+15$
404. $b^2+10b+21$
405. $x^2+11x+24$
406. $x^2-5x+4$
407. $a^2-6a+5$
408. $y^2-9y+18$
409. $b^2-11b+30$
410. $x^2-13x+40$
411. $x^2+x-2$
412. $a^2+2a-8$
413. $y^2+3y-10$
414. $b^2+4b-21$
415. $x^2+6x-16$
416. $x^2-x-6$
417. $a^2-2a-15$
418. $y^2-3y-28$
419. $b^2-4b-32$
420. $x^2-6x-27$
421. $x^2+6xy+5y^2$
422. $a^2+7ab+10b^2$
423. $m^2-8mn+12n^2$
424. $p^2-12pq+35q^2$
425. $x^2+2xy-24y^2$
426. $a^2+3ab-40b^2$
427. $x^2-xy-30y^2$
428. $a^2-4ab-45b^2$
429. $3x^2+9x+6$
430. $2a^2-14a+24$
431. $4y^2+4y-80$
432. $-x^2+5x-6$
433. $-a^2-2a+35$
434. $x^3+6x^2+8x$
435. $ab^2-5ab-24a$
436. $x^2+7x+6$
437. $a^2+8a+7$
438. $y^2+9y+14$
439. $b^2+13b+42$
440. $x^2+15x+54$
441. $x^2-8x+15$
442. $a^2-9a+8$
443. $y^2-11y+18$
444. $b^2-12b+20$
445. $x^2-16x+63$
446. $x^2+x-6$
447. $a^2+3a-4$
448. $y^2+5y-24$
449. $b^2+7b-18$
450. $x^2+2x-48$
451. $x^2-x-2$
452. $a^2-3a-18$
453. $y^2-5y-36$
454. $b^2-2b-63$
455. $x^2-4x-60$
456. $x^2+8xy+12y^2$
457. $a^2+10ab+24b^2$
458. $m^2-13mn+42n^2$
459. $p^2-15pq+50q^2$
460. $x^2+4xy-21y^2$
461. $a^2+ab-56b^2$
462. $x^2-2xy-48y^2$
463. $a^2-3ab-54b^2$
464. $5x^2+25x+30$
465. $3a^2-30a+72$
466. $2y^2+4y-70$
467. $-x^2+8x-12$
468. $-a^2-6a+27$
469. $x^3-11x^2+28x$
470. $xy^2+9xy+14x$
471. $x^2+9x+14$
472. $a^2+10a+16$
473. $y^2+12y+35$
474. $b^2+14b+48$
475. $x^2+17x+72$
476. $x^2-9x+14$
477. $a^2-10a+21$
478. $y^2-13y+30$
479. $b^2-14b+45$
480. $x^2-15x+56$
481. $x^2+x-12$
482. $a^2+4a-5$
483. $y^2+6y-27$
484. $b^2+b-72$
485. $x^2+3x-54$
486. $x^2-x-12$
487. $a^2-4a-21$
488. $y^2-6y-40$
489. $b^2-3b-70$
490. $x^2-5x-66$
491. $x^2+9xy+20y^2$
492. $a^2+11ab+28b^2$
493. $m^2-14mn+48n^2$
494. $p^2-16pq+63q^2$
495. $x^2+5xy-36y^2$
496. $a^2+2ab-63b^2$
497. $x^2-3xy-70y^2$
498. $a^2-ab-90b^2$
499. $6x^2+18x-24$
500. $2a^2-24a+70$
550. $b^2c^2+8bc-20$
Part 2: $acx^2 + (ad+bc)x + bd$ (551-700)
551. $2x^2+5x+2$
552. $3a^2+4a+1$
553. $4y^2+9y+2$
554. $5b^2+7b+2$
555. $6x^2+7x+1$
556. $2x^2-7x+3$
557. $3a^2-8a+5$
558. $4y^2-13y+3$
559. $5b^2-16b+12$
560. $6x^2-11x+4$
561. $2x^2+x-1$
562. $3a^2+2a-5$
563. $4y^2+5y-6$
564. $5b^2+b-4$
565. $6x^2+7x-20$
566. $2x^2-x-3$
567. $3a^2-a-10$
568. $4y^2-4y-15$
569. $5b^2-8b-4$
570. $6x^2-5x-21$
571. $3x^2+7xy+2y^2$
572. $2a^2+11ab+5b^2$
573. $4m^2-15mn+9n^2$
574. $6p^2-17pq+12q^2$
575. $3x^2+xy-2y^2$
576. $5a^2+9ab-2b^2$
577. $2x^2-7xy-4y^2$
578. $6a^2-ab-12b^2$
579. $4x^2+10x+6$
580. $6a^2-2a-4$
581. $-3x^2-x+2$
582. $-5x^2+14x+3$
583. $x(2x+5)-3$
584. $a(3a-1)-10$
585. $(2x+1)(x+3)-13$
700. $2(a-b)^2+5(a-b)c-3c^2$
VI. 복잡한 식과 활용 (701-1000)
Part 1: 치환을 이용한 인수분해 (701-800)
701. $(x+y)^2+3(x+y)+2$
702. $(a-b)^2-4(a-b)+3$
703. $(2x+1)^2+2(2x+1)-8$
704. $(x-2y)^2-(x-2y)-12$
705. $2(a+1)^2-5(a+1)-3$
706. $3(x-y)^2+2(x-y)-5$
707. $(x+4)^2-49$
708. $16-(a-b)^2$
709. $(2x+1)^2-(x-1)^2$
710. $4(a-b)^2-9(a+b)^2$
711. $(x^2+x-3)(x^2+x+1)+4$
712. $(x-1)(x-2)(x-3)(x-4)-24$
713. $(a-b)^2+6(a-b)+9$
714. $(3x-2y)^2-10(3x-2y)+25$
715. $4(m+n)^2+4(m+n)+1$
800. $(a^2-2a)^2-2(a^2-2a)-3$
Part 2: 여러 항의 인수분해 (801-900)
801. $ax+bx+ay+by$
802. $xy-x-y+1$
803. $a^3-a^2+a-1$
804. $x^2-y^2+x-y$
805. $4a^2-b^2-2a-b$
806. $x^2+2x+1-y^2$
807. $a^2-b^2-4b-4$
808. $x^2-6x+9-y^2$
809. $1-a^2-2ab-b^2$
810. $x^2-y^2-z^2-2yz$
811. $x^2+xy+x-2y-6$
812. $a^2-ab-4a+2b+4$
813. $m-n-m^2+n^2$
814. $xy+2z-xz-2y$
815. $a^3+2a^2-a-2$
900. $ab(a+b)+bc(b+c)+ca(c+a)+2abc$
Part 3: 인수분해 공식의 활용 (901-1000)
901. $78^2-22^2$
902. $101^2-2\times101+1$
903. $25\times 17 – 25\times 7$
904. $\sqrt{52^2-48^2}$
905. $\frac{99^2+198+1}{100}$
906. $x=\sqrt{5}-1$일 때, $x^2+2x+1$의 값을 구하시오.
907. $a=\sqrt{2}, b=2\sqrt{2}$일 때, $(a-b)^2+4ab$의 값을 구하시오.
908. $x+y=4, x-y=2\sqrt{3}$일 때, $x^2-y^2$의 값을 구하시오.
909. $x-y=3, xy=1$일 때, $x^2+y^2$의 값을 구하시오.
910. $a=\frac{1}{\sqrt{3}-\sqrt{2}}, b=\frac{1}{\sqrt{3}+\sqrt{2}}$일 때, $a^2-b^2$의 값을 구하시오.
911. $1^2-2^2+3^2-4^2$을 계산하시오.
912. $x=19$일 때, $x^2+2x-35$의 값을 구하시오.
913. $a=5.5, b=4.5$일 때, $a^2-2ab+b^2$의 값을 구하시오.
914. $x-y=5$일 때, $x^2-2xy+y^2+x-y$의 값을 구하시오.
915. $2x-y=3$일 때, $4x^2-4xy+y^2$의 값을 구하시오.
1000. $x^2-y^2=8, x-y=2$일 때, $x^2+y^2+2xy-3(x+y)$의 값을 구하시오.