A) TM MI^EN MA TRONG DO NGHI^E.M CU'A BAI TOAN CAUCHY CU'A PHU.O.N...
30)
a) Tm mi^en ma trong do nghi^e.m cu'a bai toan Cauchy cu'a phu.o.ng trnh
sau d^ay t^on ta.i va duy nh^at
y0
=√x−y.b) Tm tch ph^an t^o'ng quat cu'a cac phu.o.ng trnh sau:
(x2
−y2
)dy−2xydx= 0.HD gia’i:a) Bai toan Cauchy co duy nh^at nghi^e.m trong mi^en
D={(x, y)∈R2
|x−y≥δ}vo.i
δ >0tuy y.
b) D - u.a phu.o.ng trnh v^e da.ng
dxdy = x2
xy−y2
. D - ^ay la phu.o.ng trnh da'ng c^ap, ta da.t
z= yx. Khi do phu.o.ng trnh tr^en tro.' thanh
xz0
= z(1 +z2
)1−z2
.Hay
(1z − 2z1 +z2
)dz = dxx.
Suy ra nghi^e.m cu'a phu.o.ng trnh nay la
1 +zz2
=Cx, C 6= 0.V^a.y nghi^e.m cu'a phu.o.ng trnh da~ cho la
x2
+y2
=C1
y, C1
6= 0.