- A.10000件
- B.10500件
- C.11000件
- D.11500件
- A.甲方案可行
- B.乙方案可行
- C.丙方案可行
- D.甲、乙、丙三方案均不可行
- A.提高功能
- B.提高设计方案的实用性
- C.提高价值
- D.提高设计方案的可靠性
- A.①—③—②—④—⑤—⑥
- B.②—①—③—④—⑤—⑥
- C.②—①—③—⑤—④—⑥
- D.③—②—①—⑤—④—⑥
- A.保持产品功能不变,提高产品成本
- B.在产品成本不变的条件下,降低产品的功能
- C.运用新技术,革新产品,既提高功能又降低成本
- D.产品成本虽有增加,但功能提高的幅度更大,相应提高产品的价值
- A.1363.6
- B.1309.1
- C.13090.9
- D.13636.4 表 题114可能需要的数据(i=12%)
- A.1.19、9.19
- B.1.48、9.48
- C.1.23、9.23
- D.1.08、8.08
- A.1.54%
- B.4.51%
- C.15.8%
- D.15.4%
- A.8.26
- B.7.86
- C.7.44
- D.6.63
- A.6200
- B.6209
- C.6210
- D.6290
- A.增大
- B.不变
- C.减小
- D.不能确定
- A.<IMG src="//img1.yqda.net/question-name/3c/c2a4ea1053094a6fbcd458154ede05.jpg">
- B.<IMG src="//img1.yqda.net/question-name/13/d0b46f72c67908d723ff097b423876.jpg">
- C.<IMG src="//img1.yqda.net/question-name/e6/f19f667d0f18b162b102b205f0431b.jpg">
- D.<IMG src="//img1.yqda.net/question-name/2a/0268216505071f89f0cd3c2d0d6637.jpg">
- A.可以,该等效电源是理想电压源
- B.可以,该等效电源是理想电流源
- C.不可以等效
- D.不能确定
- A.2A的理想电流源
- B.2V的理想电压源
- C.不能确定
- D.不能代替,仍为原电路
- A.<IMG src="//img1.yqda.net/question-name/93/69b43631199cfc46494660f3935d06.jpg">
- B.<IMG src="//img1.yqda.net/question-name/f7/28a5578eac74c0d1bdec35fca6112e.jpg">
- C.<IMG src="//img1.yqda.net/question-name/ee/7374ea2d5af049d13a2d5f108f5cb1.jpg">
- D.<IMG src="//img1.yqda.net/question-name/cc/f23979b87fb1075fa5d2ddec62a192.jpg">
- A.图(a)表达式错误
- B.图(b)表达式错误
- C.图(c)表达式错误
- D.全都正确
- A.只与该电荷系所有电荷的代数和有关,而与闭合曲面的形状无关
- B.既与该电荷系所有电荷的代数和有关,也与闭合曲面的形状有关
- C.与该电荷系电荷的代数和以及闭合曲面的形状均无关
- D.只与闭合曲面的形状有关,而与该电荷系所有电荷的代数和无关
- A.1
- B.2
- C.3
- D.4
- A.4A
- B.5A
- C.2A
- D.3A
- A.1
- B.2
- C.3
- D.4
- A.100V
- B.-100V
- C.144V
- D.-144V
- A.1A
- B.5A
- C.10A
- D.15A
- A.7.0
- B.5.0
- C.6.0
- D.8.0
- A.0.0
- B.1.0
- C.2.0
- D.3.0
- A.1.0
- B.1.5
- C.2.0
- D.3.0
- A.1
- B.2
- C.3
- D.4
- A.实型,整型,实型,整型
- B.实型,整型,整型,实型
- C.实型,实型,实型,整型
- D.整型,实型,整型,实型
- A.Windows 98允许在同一个目录中,存在两个相同汉字组成的长名文件
- B.Windows 98允许在同一个目录中,存在两个相同的长名文件
- C.Windows 98允许在同一个目录中,存在同名的文件和目录
- D.Windows 98允许在同一个目录中,存在主名相同的文件
- A.计算方式简单
- B.表示形式单一规整
- C.避免与十进制相混淆
- D.与逻辑电路硬件相适应
- A.1101
- B.2325
- C.7286
- D.435
- A.101110101111
- B.110110101111
- C.101110111111
- D.111011011111
- A.CPU不能向ROM随机写入数据
- B.ROM中的内容在断电后不会消失
- C.ROM是只读存储器的英文缩写
- D.ROM是只读的,所以它不是内存而是外存
- A.<IMG src="//img1.yqda.net/question-name/78/c98f46963267f24ad23d4d199d523c.jpg">
- B.△h
- C.<IMG src="//img1.yqda.net/question-name/26/972b8c71f9989f999ad3f4bc59101a.jpg">
- D.2△h
- A.重力和压力之比
- B.重力和黏性力之比
- C.惯性力和重力之比
- D.惯性力和黏性力之比
- A.h<SUB>1</SUB>=h<SUB>2</SUB>,与容器内液面平齐
- B.h<SUB>1</SUB>>h<SUB>2</SUB>
- C.h<SUB>1</SUB>=h<SUB>2</SUB>,高于容器内液面
- D.h<SUB>1</SUB><h<SUB>2</SUB>
- A.2.0
- B.0.5
- C.1.0
- D.4.0
- A.只有速度头v<SUP>2</SUP>/(2g)可忽略不计
- B.只有局部水头损失h<SUB>i</SUB>均可忽略不计
- C.速度头v<SUP>2</SUP>/(2g)和局部水头损失h<SUB>i</SUB>均可忽略不计
- D.两断面的测压管水头差△H<SUB>p1-2</SUB>并不等于两断面间的沿程水头损失h<SUB>f1-2</SUB>
- A.增大
- B.减小
- C.不变
- D.不定
- A.压差计
- B.毕托管
- C.文丘里管状水表
- D.孔板流量计
- A.577
- B.688
- C.775
- D.885
- A.1027.2
- B.942.5
- C.946.4
- D.950.2
- A.8.781 L/s
- B.7.605 L/s
- C.9.818 L/s
- D.6.209 L/s
- A.9.8kN
- B.19.6kN
- C.38.6kN
- D.54.0kN
- A.液体的黏滞系数随温度的升高而增大
- B.流体的黏性是产生能量损失的根源
- C.黏性就是流体具有抵抗剪切变形的能力
- D.流体具有内摩擦力的特性就是流体的黏性
- A.8倍
- B.7倍
- C.5倍
- D.2倍
- A.<IMG src="//img1.yqda.net/question-name/05/77574610c29ae207edaeaacda0a882.jpg">
- B.<IMG src="//img1.yqda.net/question-name/7e/88ba2a4a095a84ec1d2c9941e08ce4.jpg">
- C.<IMG src="//img1.yqda.net/question-name/f2/90ae95987540457c14e335b4ccc4c3.jpg">
- D.<IMG src="//img1.yqda.net/question-name/91/ca7ba0844d0ca9c130161f727fac31.jpg">
- A.0
- B.(1-2v)σ/E
- C.3(1-2v)σ/E
- D.(1-2v)σ<SUP>2</SUP>/E
- A.a梁和b梁的最大挠度相同
- B.b梁的最大挠度是a梁的2倍
- C.b梁的最大挠度是a梁的4倍
- D.b梁的最大挠度是a梁的8倍
- A.a点
- B.b点
- C.c点
- D.d点
- A.25:9
- B.5:3
- C.3:5
- D.9:25
- A.<IMG src="//img1.yqda.net/question-name/d1/c8d811f99b4a5a7fbdd553c9843a26.jpg">
- B.<IMG src="//img1.yqda.net/question-name/5d/9d210485b7600c6b8022af6f0c25a8.jpg">
- C.<IMG src="//img1.yqda.net/question-name/43/45a7597f2e0ae87bf78a7046e78c2b.jpg">
- D.<IMG src="//img1.yqda.net/question-name/de/d1b878bf808aee1bc88842c42d0320.jpg">
- A.5a/6
- B.6a/5
- C.6a/7
- D.17a/6
- A.(I<SUB>z</SUB>)<SUB>a</SUB>=(I<SUB>z</SUB>)<SUB>b</SUB>,(I<SUB>y</SUB>)<SUB>a</SUB>=(I<SUB>y</SUB>)<SUB>b</SUB>
- B.(I<SUB>z</SUB>)<SUB>a</SUB>>(I<SUB>z</SUB>)<SUB>b</SUB>,(I<SUB>y</SUB>)<SUB>a</SUB>>(I<SUB>y</SUB>)<SUB>b</SUB>
- C.(I<SUB>z</SUB>)<SUB>a</SUB>>(I<SUB>z</SUB>)<SUB>b</SUB>,(I<SUB>y</SUB>)<SUB>a</SUB>=(I<SUB>y</SUB>)<SUB>b</SUB>
- D.(I<SUB>z</SUB>)<SUB>a</SUB>=(I<SUB>z</SUB>)<SUB>b</SUB>,(I<SUB>y</SUB>)<SUB>a</SUB>>(I<SUB>y</SUB>)<SUB>b</SUB>
- A.图(a)点应力状态较危险
- B.图(b)应力状态较危险
- C.两者的危险程度相同
- D.不能判定
- A.0.036mm
- B.0.018nun
- C.0.18mm
- D.0.36mm
- A.(1-α<sup>4</sup>)<sup>3/2</sup>
- B.(1-α<sup>2</sup>)(1-α<sup>4</sup>)<sup>3/2</sup>
- C.(1-α<sup>4</sup>)(1-α<sup>2</sup>)
- D.(1-α<sup>4</sup>)<sup>3/2</sup>(1-α<sup>2</sup>)
- A.-20 MPa,-80 MPa
- B.20MPa,-80 MPa
- C.80 MPa,-20 MPa
- D.60 MPa,-30 MPa
- A.0
- B.<IMG src="//img1.yqda.net/question-name/93/677637cd516928abf69739b6fc17c6.jpg">
- C.<IMG src="//img1.yqda.net/question-name/07/81d1e4720d41febb7b6a945c98488b.jpg">
- D.<IMG src="//img1.yqda.net/question-name/98/07f7a9857d46fe8e9b368207e100a2.jpg">
- A.[F]=A[σ]
- B.[F]=2A[σ]
- C.[F]=3A[σ]
- D.[F]=4A[σ]
- A.<img src="//img1.yqda.net/question-name/90/18b511a128999dd36c9deb7418fa8d.jpg">
- B.<img src="//img1.yqda.net/question-name/10/337f5c659928847631bf860c9d335f.jpg">
- C.<img src="//img1.yqda.net/question-name/66/3f66ce0744f477f7bc29f9426c3e65.jpg">
- D.<img src="//img1.yqda.net/question-name/30/2b902b51693e93a5b3056dc5b96bb4.jpg">
- A.<IMG src="//img1.yqda.net/question-name/df/caab41b75a1b3c704b5f2a6d84c5e2.jpg">
- B.<IMG src="//img1.yqda.net/question-name/3b/4670a953e84aa6832647cd80e20fb2.jpg">
- C.P
- D.0.5P
- A.<IMG src="//img1.yqda.net/question-name/b6/423192ac6864c44a54bc6b3ba22dc7.jpg">
- B.<IMG src="//img1.yqda.net/question-name/84/2f6d3e5996d26dd82fe20b0151834c.jpg">
- C.<IMG src="//img1.yqda.net/question-name/3d/861a6796f78a8d6f3752f78752a236.jpg">
- D.<IMG src="//img1.yqda.net/question-name/71/0e72547980f1319ee88db732c47823.jpg">
- A.T=W
- B.T=2W
- C.<IMG src="//img1.yqda.net/question-name/9f/b408045b944a3fe4673ec82217c365.jpg">
- D.T=W/2
- A.α=30°,β=45°
- B.α=45°,β=305°
- C.α=45°,β=135°
- D.α=135°,β=45°
- A.<IMG src="//img1.yqda.net/question-name/32/da1e9668d86bea00502ae52d0123cf.jpg">
- B.<IMG src="//img1.yqda.net/question-name/c3/cf89fbac5f8c8ca34146c6b90fa620.jpg">
- C.<IMG src="//img1.yqda.net/question-name/0c/b4464a7f134eb9c1445fd8a371cc98.jpg">
- D.<IMG src="//img1.yqda.net/question-name/1d/4b7f72c89e54379729060175085d93.jpg">
- A.越来越小
- B.越来越大
- C.匀速运动,a无变化
- D.条件不足,难以判断
- A.<IMG src="//img1.yqda.net/question-name/08/c91078f7add4bd290b27ccb87505ac.jpg">
- B.<IMG src="//img1.yqda.net/question-name/2e/430250e9d517224b4da1d6c15a331c.jpg">
- C.<IMG src="//img1.yqda.net/question-name/79/dd8aece302adff1991d11165aa3cdc.jpg">
- D.<IMG src="//img1.yqda.net/question-name/ce/6dbfb60a3bf150724366e526b1991c.jpg">
- A.<IMG src="//img1.yqda.net/question-name/55/4ef3e7108ca2eb7a7d96af96d08c13.jpg">
- B.<IMG src="//img1.yqda.net/question-name/9d/85ebe78c5dbf508d137e0e15cae431.jpg">
- C.<IMG src="//img1.yqda.net/question-name/ff/7ac8c42a63e69b200ceff3db4cbb80.jpg">
- D.<IMG src="//img1.yqda.net/question-name/db/a78d1a8f29c7ed6fc0fdd981506ebc.jpg">
- A.正确。无水平载荷,A、B处反力应无水平分量
- B.错误。受力图中未画出铰链C的反力
- C.错误。AC和BC构件均为二力构件,A、B处反力应有水平分量
- D.错误。A、B处有水平分力,受力图中未画出
- A.R<SUB>A</SUB>=0
- B.E<SUB>A</SUB>=50kN(→)
- C.R<SUB>A</SUB>=50kN(←)
- D.R<SUB>A</SUB>=100kN(↑)
- A.聚异戊二烯
- B.1,4-聚丁二烯
- C.1,3-丁二烯
- D.苯乙烯
- A.2,3-二甲基-1-丁烯
- B.2-戊烯
- C.2-甲基-2-丁烯
- D.丙烯
- A.R<SUB>A</SUB>沿铅直线
- B.R<SUB>A</SUB>沿A、B连线
- C.R<SUB>A</SUB>沿A、C连线
- D.R<SUB>A</SUB>平行于B、C连线
- A.H<SUB>2</SUB>O
- B.H<SUB>2</SUB>S
- C.H<SUB>2</SUB>Se
- D.H<SUB>2</SUB>Te
- A.sp<SUP>3</SUP>杂化
- B.sp杂化
- C.sp<SUP>2</SUP>杂化
- D.sp<SUP>3</SUP>不等性杂化
- A.1s<SUP>2</SUP>2s<SUP>2</SUP>2p<SUP>54</SUP>f<SUP>1</SUP>
- B.1s<SUP>2</SUP>2P<SUP>2</SUP>
- C.1s<SUP>2</SUP>2s<SUP>1</SUP>2P<SUP>1</SUP>
- D.1s<SUP>2</SUP>2s<SUP>2</SUP>
- A.增大
- B.减小
- C.不变
- D.不能判断
- A.<IMG src="//img1.yqda.net/question-name/2c/f2c08353fc91f1e27788db9d47edde.jpg">
- B.<IMG src="//img1.yqda.net/question-name/05/f987a130ec6f09880393f5446f6914.jpg">
- C.<IMG src="//img1.yqda.net/question-name/c9/4c39b54030968cddddb81938a31fce.jpg">
- D.<IMG src="//img1.yqda.net/question-name/7c/25f9ef77e23dcc65fc4c7efa22b96d.jpg">
- A.NaCl
- B.K<SUB>2</SUB>CO<SUB>3</SUB>
- C.Na<SUB>2</SUB>SO<SUB>4</SUB>
- D.Na<SUB>2</SUB>S
- A.0.2mol·L<SUP>-1</SUP>NH<SUB>3</SUB>·H<SUB>2</SUB>O与0.1 mol·L<SUP>-1</SUP>HCl等体积混合
- B.0.2mol·L<SUP>-1</SUP>HCl与0.1mol·L<SUP>-1</SUP>NH<SUB>3</SUB>·H<SUB>2</SUB>O等体积混合
- C.0.1mol·L<SUP>-1</SUP>NaAc与0.2mol·L<SUP>-1</SUP>HCl等体积混合
- D.0.1mol·L<SUP>-1</SUP>NH<SUB>3</SUB>·H<SUB>2</SUB>O与0.1mol·L<SUP>-1</SUP>HCI等体积混合
- A.y=3cos(4πt-x/20)
- B.y=3COS4π(t+x/20)
- C.y=3COS 4π(t-x/20)
- D.y=3cos(4πt+x/20)
- A.反应物不能变成产物
- B.系统因反应而温度下降
- C.不能判断
- D.系统因反应而温度上升
- A.5条
- B.7条
- C.9条
- D.10条
- A.<IMG src="//img1.yqda.net/question-name/7f/9033dfe43f55df43a97164e58e82e6.jpg">
- B.λ/2
- C.λ
- D.λ/n
- A.I<SUB>0</SUB>/8
- B.3I<SUB>0</SUB>/8
- C.I<SUB>0</SUB>/4
- D.3I<SUB>0</SUB>/4
- A.干涉条纹的宽度将发生改变
- B.产生红光和蓝光的两套彩色干涉条纹
- C.干涉条纹的亮度将发生改变
- D.不产生干涉条纹
- A.1.02×10<SUP>-6</SUP>m
- B.2.04×10<SUP>-6</SUP>m
- C.2.04×10<SUP>-7</SUP>m
- D.1.02×10<SUP>-7</SUP>m
- A.宽度变小
- B.宽度变大
- C.宽度不变,且中心强度也不变
- D.宽度不变,但中心强度变小
- A.2个
- B.4个
- C.6个
- D.8个
- A.π/2
- B.π/3
- C.π/6
- D.<IMG src="//img1.yqda.net/question-name/a3/1e70ff610f6d8ccffdd506179b80fa.jpg">
- A.膨胀后,温度不变,压强减小
- B.膨胀后,温度降低,压强减小
- C.膨胀后,温度升高,压强减小
- D.膨胀后,温度不变,压强增加
- A.平均碰撞次数减小而平均自由程不变
- B.平均碰撞次数减小而平均自由程增大
- C.平均碰撞次数增大而平均自由程减小
- D.平均碰撞次数不变而平均自由程增大
- A.PV/m
- B.PV/(KT)
- C.PV/(RT)
- D.PV(mT)
- A.0
- B.<IMG src="//img1.yqda.net/question-name/0c/893d7c4c0232a3af87388a33d9b19b.jpg">
- C.<IMG src="//img1.yqda.net/question-name/de/d919928671ea8ffba5f76f78adeb89.jpg">
- D.1
- A.2,0.4
- B.1,0.4
- C.3,0.6
- D.4,0.6
- A.1
- B.2
- C.-2
- D.-1
- A.<IMG src="//img1.yqda.net/question-name/65/4905492f5b6c72e02f0ca9a8e207d6.jpg">
- B.<IMG src="//img1.yqda.net/question-name/e3/57b2e55f94311957c7d42b785f4c36.jpg">
- C.<IMG src="//img1.yqda.net/question-name/66/63855588f359e34b683a296ce0374a.jpg">
- D.<IMG src="//img1.yqda.net/question-name/87/41f4f9768a996df40dff689862a8ed.jpg">
- A.1/3
- B.2/3
- C.1/4
- D.1/2
- 102
-
行列式
的值为( )。
- A.3
- B.4
- C.5
- D.6
- A.2
- B.-7
- C.2,-7
- D.不存在
- A.<img src="//img1.yqda.net/question-name/51/5688af4fbeeb6a85d7232a20da4f01.jpg">
- B.<img src="//img1.yqda.net/question-name/e0/214173c19917b7410d0b681b978006.jpg">
- C.<img src="//img1.yqda.net/question-name/0c/2e8d48677c8bc8db2af612a6fd6383.jpg">
- D.<img src="//img1.yqda.net/question-name/1f/f7a46942acbafc4cd200a766b4a004.jpg">
- A.y=e<SUP>-sinx</SUP>
- B.y=xe<SUP>-sinx</SUP>
- C.y=ce<SUP>-sinx</SUP>
- D.y=e<SUP>-sinx</SUP>(C+X)
- A.Ax<SUP>2</SUP>e<SUP>-x</SUP>
- B.x<SUP>2</SUP>e<SUP>-x</SUP>
- C.(Ax+B)xe<SUP>-x</SUP>
- D.(Ax+B)x<SUP>2</SUP>e<SUP>-x</SUP>
- A.<IMG src="//img1.yqda.net/question-name/c7/d75e054c515d2a893677b43dc44808.jpg">
- B.3
- C.2
- D.<IMG src="//img1.yqda.net/question-name/f9/16c9d4ad2a827d17a97c1235c807ed.jpg">
- 108
-
下列级数中,发散的是( )。
- A.<IMG src="//img1.yqda.net/question-name/a3/0be2afad9cc5e83fcbb46bc5b60a31.jpg">
- B.<IMG src="//img1.yqda.net/question-name/86/1fd3e7b51e6953664652e099a7d2d8.jpg">
- C.<IMG src="//img1.yqda.net/question-name/ff/c4e7269049fef5c3e80df77ad0fbe2.jpg">
- D.<IMG src="//img1.yqda.net/question-name/11/78feaf62498471127ede135673d756.jpg">
- A.x-1=y-1=z-1
- B.<IMG src="//img1.yqda.net/question-name/4e/e3bce3a463e8e4d58c71ceafed592d.jpg">
- C.<IMG src="//img1.yqda.net/question-name/7d/c88f759bb776b66ed23ca153f3a9ca.jpg">
- D.x+1=y+1=z+1
- A.<IMG src="//img1.yqda.net/question-name/1a/28bcfd7825c3dd1300aa09a7707e72.jpg">
- B.<IMG src="//img1.yqda.net/question-name/25/07637d8801cbf544fcecfc83f6a0ae.jpg">
- C.<IMG src="//img1.yqda.net/question-name/58/d1131af23d610d046734cf2ba655b8.jpg">
- D.<IMG src="//img1.yqda.net/question-name/98/a1d6a79a737f3e32d3a13af729ec8c.jpg">
- A.<IMG src="//img1.yqda.net/question-name/cc/8221ab6cc6a5f278cb842f50d959c3.jpg">
- B.<IMG src="//img1.yqda.net/question-name/18/2d54f1d794204e68527183c8406c7b.jpg">
- C.<IMG src="//img1.yqda.net/question-name/45/c497cfc871009249643d2bb5f5e44c.jpg">
- D.<IMG src="//img1.yqda.net/question-name/94/11f6d27f83445fc54021d1e58a9b0e.jpg">
- A.3x-4z+1=0
- B.3x+z/4+1=0
- C.<IMG src="//img1.yqda.net/question-name/e0/b8d28437a6c1b5574cf3c23191bd95.jpg">
- D.3x+4z+1=0
- 113
-
下列等式中正确的是( )。
- A.a(a·b)=|a|<SUP>2</SUP>b
- B.(a·b)<SUP>2</SUP>=|a|<SUP>2</SUP>|b|<SUP>2</SUP>
- C.(a+b)×(a-b)=a×a-b×b
- D.(a+b)·(a-b)=|a|<SUP>2</SUP>-|b|<SUP>2</SUP>
- A.<IMG src="//img1.yqda.net/question-name/f4/5c5ebb6a9e57966774dce02b9c1262.jpg">
- B.<IMG src="//img1.yqda.net/question-name/c6/4c06c0d1ec0866cf8d88c7e78e5806.jpg">
- C.<IMG src="//img1.yqda.net/question-name/45/8c46e944b93c14c699489644ee0d2c.jpg">
- D.<IMG src="//img1.yqda.net/question-name/72/f10d2bedbd475e245bef7025d22107.jpg">
- A.<IMG src="//img1.yqda.net/question-name/ad/2ab70aeb9f2b34f6e48568b506f89d.jpg">
- B.<IMG src="//img1.yqda.net/question-name/d1/b6c55860eeacf4aa5889486caaa566.jpg">
- C.<IMG src="//img1.yqda.net/question-name/1c/862e3280a5aef606c6c1ecb04215ea.jpg">
- D.<IMG src="//img1.yqda.net/question-name/42/c79b2c7b4a6ff1f762e95ce4bed282.jpg">
- 119
-
设
,则下列结论正确的是( )。
- A.f(x<SUB>0</SUB>)不是f(x)的极值,(x<SUB>0</SUB>,f(x<SUB>0</SUB>))不是曲线的拐点
- B.f(x<SUB>0</SUB>)是f(x)的极值,(x<SUB>0</SUB>,f(x<SUB>0</SUB>))不是曲线的拐点
- C.f(x<SUB>0</SUB>)不是f(x)的极值,(x<SUB>0</SUB>,f(x<SUB>0</SUB>))是曲线的拐点
- D.f(x<SUB>0</SUB>)是f(x)的极值,(x<SUB>0</SUB>,f(x<SUB>0</SUB>))是曲线的拐点