GIA'I PHU.O.NG TRNH VI PH^AN
172)
Gia'i phu.o.ng trnh vi ph^an:
y”−(2ex
+ 1)y0
+e2x
y=e3x
bang phep d^o'i bi^en
t=ex
.
HD gia’i:D - ^o'i bi^en
t=ex
⇒yx
0
=yt
0
.ex
, y”xx
=y”tt
.e2x
+yt
0
.ex
Thay vao phu.o.ng trnh:
y”tt
−2y0
t
+y=t3
Nghi^e.m t^o'ng quat cu'a phu.o.ng trnh thu^an nh^at:
y=et
(C1
t+C2
)Tm nghi^e.m ri^eng da.ng
y = At3
+Bt2
+Ct+D → y = t3
+ 6t2
+ 18t+ 24K^et qua'
y=ee
x
(C1
ex
+C2
) +e3x
+ 6e2x
+ 18ex
+ 24.