作者goshfju (cola)
看板FJU_CLASS
标题[考题] 日/数学系/叶乃实/微分方程
时间Wed Jan 24 23:17:57 2007
上学期期中考(答案全打要很久 所以只写大方向喔)
1.Consider dy/dx= -(x^2)y , and answer the following questions:
(i)please draw the isoclines
(ii)please draw the direction field
(iii)attempt the approximate integral curves
(iv)in case y(0)=1, show that there is a unique solution
by NOT solving the problem
(i)(ii)(iii)图描仔细就好
(iv) f(x,y)=-(x^2)y
⊿f/⊿y=-x^2 is continuous near x=0
by the uniqueness and existence thm.
there is only one sol.
2.xy'(3(x^2)+2y)- 5y((x^2)+y) - (x^4) =0
Please solve y(x).
f(x,y,y')=xy'(3(x^2)+2y)- 5y((x^2)+y) - (x^4)
f(tx,(t^m)y,(t^m-1)y')=(t^r)f(x,y,y')
could find it's a isobraic fn. with r=4 m=2
f(1,y/x^2,y/x^4)= f(x,y,y')/x^4 = (3/x)y' + (2/x^3)yy' - (5/x^2)y
- 5(y^2/x^4) -1 ----(1)
let u=y/x^2 thus y=u(x^2) , y'=2ux+(x^2)u' substitute into (1)
...
整理後得 xu'=(u^2-u+1)/(2u+3)
(1/x)dx = (2u+3)/(u^2-u+1) du
...
积分後得 ln|cx/(u^2-u+1)|=(8/√3)arctan((2u-1)/√3)
ln|(cx^5)/(y^2-(x^2)y+x^4)|=(8/√3)arctan((2(y/x^2)-1)/√3)
3.Find the orthogonal trajectories of the curves represented by the
differential equation r^n=(a^n)sec(nθ) , where n is an positive integer
and a is an arbitrary const.
r^n=(a^n)sec(nθ)
n(r^(n-1))r' = n(a^n)sec(nθ)tan(nθ)
tanΨf= r/r' = cot(nθ)
tanΨg= -1/tanΨf =-tan(nθ)= rg/rg'= rg/(drg/dθ)
drg/rg=-cot(nθ)dθ
ln|rg|= -(1/n)ln|sin(nθ)|+c
rg^n=csc(nθ) :orthogonal trajectories
4.y=y(x) and y'=(-1/x)+y/x+(y^2)/(x^3)
please find the solution of y
y=x and y=-x are 2 PSs
y'=(-1/x)+y/x+(y^2)/(x^3) ...(1)
x'=-1/x+x/x+(x^2)/(x^3) ...(2)
(-x)'=-1/x+(-x/x)+(-x)^2/x^3 ...(3)
(1)-(2) (y-x)'=(y-x)/x + (y^2-x^2)/x^3
(y-x)'/(y-x) = 1/x + (y+x)/x^3 ...(4)
(1)-(3) (y+x)'/(y+x) = 1/x + (y-x)/x^3 ...(5)
(4)-(5) (y-x)'/(y-x) - (y+x)'/(y+x) = 2/x^2
ln|(y-x)/c(y+x)|= -2/(x^2)
y= x[1+ce^(-2/x)]/[1-ce^(-2/x)]
5.(x^2+siny)y'+2xy=0 please solve y(x)
2xydx + (x^2+siny)dy
M=2xy N=x^2+siny
Mx=Ny=2x ...exact
x y
u=∫ 2tYodt + ∫ (Xo^2+sint)dt=c
Xo Yo
Let Xo=0 Yo=π/2
(x^2)y-cosy=c
就这样啦@@""用BBS打数学式子真是噩梦啊
这是以前的题目..今年我没把题目抄下来...不过也是大同小异啦
老师每年都会出像第一题的做图题..
还有第二题arctan的积分会积出很丑的答案..今年也有一样的..
其实期中考的题目都蛮基本的 跟讲义里的题目类似
期末考会比期中难非常多倍...想保住不被当的话..期中考高点吧@@""
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※ 发信站: 批踢踢实业坊(ptt.cc)
◆ From: 218.167.79.24
1F：→ goshfju:今年的题目有一题也是求orthogonal trajectories 01/24 23:22
2F：推 fm1210:真是太辛苦了 哇 01/24 23:23
3F：→ goshfju:有一题是primitive=>D.E. 01/24 23:23
4F：→ goshfju:其实是太穷了 来骗P币的 XD 01/24 23:26
5F：→ goshfju:有一题是bernoulli 01/24 23:26
6F：→ goshfju:还有一题忘记了@@"想到再推 01/24 23:29
※ 编辑: goshfju 来自: 218.167.79.24 (01/24 23:33)