高等数学·极限

极限

极限的定义

1)数列极限

(1)limnxn=Aϵ0,\existN,使nN,|xnA|<ϵ(2)limxf(x)=Aϵ0,\existM0,使|x|M,|f(x)A|<ϵ(3)limx+f(x)=Aϵ0,\existM0,使xM,|f(x)A|<ϵ(4)limxf(x)=Aϵ0,\existM0,使x<M,|f(x)A|<ϵ(5)limxx0f(x)=Aϵ0,\existM0,使xM,|f(x)A|<ϵ

极限的性质

1)局部保号性

(6)limxx0f(x)=A0(<0),(7)\existδ0,使xU0(x0,δ),f(x)0(<0)

推论:保序性:

(8)limxx0f(x)=A0(<0),α<A(βA),(9)\existδ0,使xU0(x0,δ),f(x)α(f(x)<β)

2)局部有界性

(10)limxx0f(x)=A,\existU0(x0),使f(x)U0(x0)

3)不等式性质

(11)limxx0f(x)=A,(12)limxx0g(x)=B,(13)f(x)g(x),(14)AB(15)(16)f(x)g(x)f(x)g(x),AB(17)x+,1x1x+1limx+1xlimx+1x+1(18)limx+1xlimx+1x+1

推论:

limxx0,f(x)0(0),A0(A0)

4)四则运算

(19)limf(x)=A,limg(x)=B,(20)lim[f(x)±g(x)]=A±B(21)limf(x)g(x)=AB(22)limf(x)g(x)=AB,(B0)(23)(24)limf(x),limg(x)=B,(25)lim[f(x)±g(x)],(26)limf(x)g(x)(27)

数列极限

(28)1limnxn=A:(29)ϵ0,\existN0,nN,|xnA|<ϵ(30)(31)(1)ϵN(32)ϵxnA,Nn(33)(2)(34)ϵϵn(Aϵ,A+ϵ)

img

(35)(3)

img

(36)(4)limnxn=alimkx2k1=limkx2k=a:(37)\exist\exist(38)\exist\exist=\exist(39)eg:an=(1)n,a2k1=1,1,1,...,1;a2k=1,1,1,...,1;limka2k1limka2k

例题1

img

(40)1(41)limn(n+1n)1=1(42)limn(n+1n)1=1(43)2+(44)(n+1n)1(n+1n)(1)nn+1n(45)limn(n+1n)1=1limn(n+1n)(1)nlimnn+1n=1(46)I=limn(n+1n)(1)n=1

例题2

eqfdsfas1312dsafdwftht

(47)(1):||a||b|||ab|(48)limnxn=a(49)ϵ0,\existN0,nN,|xna|<ϵ(50)||xn||a|||xna|,(51)ϵ0,\existN0,nN,||xn||a||<ϵ(52),xn=(1)n,limn|xn|=1=|1|,limn(1)n(53)(2)(1),limnxn=0,limn|xn|=|0|=0(54)limn|xn|=0,ϵ0,\existN0,nN,||xn|0|<ϵ(55)|xn0|<ϵ

求数列极限的方法:

(56)1

函数极限

1)自变量趋于无穷大时函数的极限

image-20210428170621601

例题

(57)limxx2+1x=?(58)(59)x2=|x|(60)(61)limx+x1+1x2x=1(62)limxx1+1x2x=1(63)limxx1+1x2xlimx+x1+1x2x(64)limxx2+1x

2)自变量趋于有限值时函数的极限

image-20210428172430632

(65)(1)ϵ,ϵδ,|f(x)A|<ϵ(66)Aϵ<f(x)<A+ϵ(67)(2)f(x0)

image-20210502180033951

易错点:

(68)limx0sinxx=1(x0,x0)(69)limx0sin(xsin1x)xsin1x=1(70)xsin1x0,xsin1x0(71)0xsin1x0(72)x=1nπ,使xsin1x=0(73)

image-20210502181857574

223567833

1619951164(1)

极限性质

(74)1)(75)(1)[xn],[xn](76)xna,nN,xnM

123

(77)xnn,Mn(78)(79)eg:xn=(1)n(80)(2)limxx0f(x)\exist,f(x)x0(81)limxx0f(x)\existf(x)()(82)eg:f(x)=sin1x,limx0sin1x,(83)2)(84)(1)(85)limnxn=A(86)[1]A0(A<0),N0,nN,xn0(xn<0)(87)[2]N0,nN,xn0(xn0),A0(A0)(88)(2)(89)[1]A0(A<0),δ0,xU˙(x0,δ),f(x)0(f(x0)<0)(90)[2]δ0,xU˙(x0,δ),f(x)0(f(x)0),A0(A0)

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