作者kagato (包)
看板Grad-ProbAsk
标题Re: [理工] [工数]-O.D.E.
时间Fri Jan 15 14:12:56 2010
※ 引述《smallprawn (水中瑕)》之铭言：
: 2
: 1.(x-1) y''+4(x-1)y'+2y=1
let x-1 = exp^t , t = ln(x-1) , D=d/dt
[D(D-1) + 4D + 2 ]y = 1
[D^2+3D+2]y = 1
yh = c1exp^(-t) + c2exp^(-2t)
yp = 1/2
y = yh + yp = c1exp^(-t) + c2exp^(-2t) + 1/2
= c1/(x-1) + c2/(x-1)^2 + 1/2
: 2
: 答案为y=(c1/(x-1))+(c2/(x-1) )+1/2
: 2 2
: 但我解出来的答案为 y=(1/(x-1) )[x +c1x+c2]
: 是我解错吗= =!?请高手解答!!
你的答案如果x^2系数是1/2应该就对@@
: 2
: 2.xy''+y'=(y'')
题目应该是 xy'' + y' = (y')^2 吧!?
let y'=p → xp' + p = p^2
(xp)' = (xp)^2 / x^2
→ d(xp)/(xp)^2 = dx/x^2
1/xp = 1/x + c1
→ 1 = p +c1xp = p(c1x+1) = y' (c1x+1)
dy = dx/(c1x+1) → y = 1/c1 *ln[c1x+1] +c2
: 答案为 y=(1/c1) ln|c1x+1|+c2
: 但我算出来为 y=(-1/c1) ln|1-c1x|+c2
: 这答案也是正确的吗= =? 还是她答案错了!?
: 请高手解答搂!!感恩!!
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1F：推 smallprawn:为什麽你的yp直接就知道是1/2= =.. 01/15 15:43
2F：推 squallting:因为等号右边是常数 假设yp=A带入 求得A=1/2 01/15 17:37