WEBVTT 00:00:00.440 --> 00:00:01.930 在这段视频中，我将向你们展示一种技巧 00:00:02.930 --> 00:00:09.310 叫做配方法。 00:00:09.310 --> 00:00:14.490 它的奇妙之处在于它适用于任何二次方程， 00:00:14.490 --> 00:00:18.210 它实际上是 00:00:18.210 --> 00:00:18.750 二次方程的基。 00:00:18.750 --> 00:00:21.990 在下个或者在那之后的视频， 00:00:21.990 --> 00:00:25.630 我会用配方法来证明这个二次公式。 00:00:25.630 --> 00:00:28.450 但在此之前，我们需要理解。 00:00:28.450 --> 00:00:29.470 它到底是怎么回事。 00:00:29.470 --> 00:00:32.070 这是建立在上一集视频基础上的， 00:00:32.070 --> 00:00:33.880 我们用完全平方 00:00:33.880 --> 00:00:36.130 求解了二次方程。 00:00:36.130 --> 00:00:39.900 已知二次方程x² - 4x 00:00:39.900 --> 00:00:44.880 等于5。 00:00:44.880 --> 00:00:47.490 我在这里放这么大的空间是有原因的。 00:00:47.490 --> 00:00:49.680 在上一集视频中，我们知道 00:00:49.680 --> 00:00:53.200 如果左边是完全平方， 00:00:53.200 --> 00:00:56.500 就可以很直接地求解出来。 00:00:56.500 --> 00:00:59.050 你看，配方就是把二次方程变成完全平方， 00:00:59.050 --> 00:01:01.900 对它进行工程处理， 00:01:01.900 --> 00:01:03.721 两边加减 00:01:03.721 --> 00:01:05.970 使它变成完全平方。 00:01:05.970 --> 00:01:07.710 我们该怎么做呢? 00:01:07.710 --> 00:01:10.130 为了使左边是完全平方的， 00:01:10.130 --> 00:01:12.990 这里必须有某个数。 00:01:12.990 --> 00:01:17.510 00:01:17.510 --> 00:01:20.910 00:01:20.910 --> 00:01:22.890 00:01:22.890 --> 00:01:24.750 00:01:24.750 --> 00:01:27.700 00:01:27.700 --> 00:01:35.230 00:01:35.230 --> 00:01:37.740 00:01:37.740 --> 00:01:41.010 00:01:41.010 --> 00:01:42.110 00:01:42.110 --> 00:01:46.180 00:01:46.180 --> 00:01:49.330 00:01:49.330 --> 00:01:53.640 00:01:53.640 --> 00:01:59.880 00:01:59.880 --> 00:02:03.530 00:02:03.530 --> 00:02:07.690 00:02:07.690 --> 00:02:10.370 00:02:10.370 --> 00:02:13.570 00:02:13.570 --> 00:02:18.330 00:02:18.330 --> 00:02:21.550 00:02:21.550 --> 00:02:24.220 00:02:24.220 --> 00:02:25.910 00:02:25.910 --> 00:02:29.080 00:02:29.080 --> 00:02:32.670 00:02:32.670 --> 00:02:35.920 00:02:35.920 --> 00:02:40.230 00:02:40.230 --> 00:02:41.720 00:02:41.720 --> 00:02:44.100 00:02:44.100 --> 00:02:46.540 00:02:46.540 --> 00:02:47.630 00:02:47.630 --> 00:02:50.990 00:02:50.990 --> 00:02:55.240 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