Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Ecuat¸ii ¸si sisteme <strong>de</strong> ecuatii diferent¸iale liniare 145<br />
Exercit¸iul 7.1 Să se <strong>de</strong>termine solut¸ia generală a ecuat¸iilor<br />
Răspuns.<br />
a) y(x) = c1 e 2x + c2 e 3x<br />
a) y ′′ − 5y ′ + 6y = 0<br />
b) y ′′′ − 6y ′′ + 12y ′ − 8y = 0<br />
c) y ′′ + y ′ + y = 0<br />
d) (D 2 − D + 1) 3 y = 0<br />
e) (D − 3) 4 (D 2 + 2) 2 y = 0.<br />
b) y(x) = c1 e 2x + c2 x e 2x + c3 x 2 e 2x<br />
1<br />
− c) y(x) = c1 e 2 x √<br />
3 cos(<br />
2 x) + c2 e<br />
− 1<br />
2 x sin(<br />
d) y(x) = c1 e 1<br />
2 x √<br />
3 cos( 2 x) + c2 e 1<br />
2 x √<br />
3 sin(<br />
1<br />
− +c3 x e 2 x √<br />
1<br />
3<br />
− cos(<br />
2 x) + c4 x e<br />
√ 3<br />
2 x)<br />
2 x)<br />
2 x sin(<br />
√ 3<br />
2 x)<br />
+c5 x2 1<br />
− e 2 x √<br />
3 cos( 2 x) + c6 x2 1<br />
− e 2 x √<br />
3 sin( 2 x)<br />
e) y(x) = c1 e 3x + c2 x e 3x + c3 x 2 e 3x + c4 x 3 e 3x<br />
+c5 cos( √ 2x) + c6 sin( √ 2x) + c7 x cos( √ 2x) + c8 x sin( √ 2x).<br />
Propozit¸ia 7.23 Ecuat¸ia Euler<br />
a0 x n y (n) + a1 x n−1 y (n−1) + · · · + an−1 x y ′ + an y = 0<br />
se reduce la o ecuat¸ie diferent¸ială liniară cu coeficient¸i constant¸i prin schimbarea <strong>de</strong><br />
variabilă x = e t , un<strong>de</strong> t este noua variabilă in<strong>de</strong>pen<strong>de</strong>ntă.<br />
Demonstrat¸ie (cazul n = 3). Notând cu z(t) noua funct¸ie necunoscută avem relat¸ia<br />
care prin <strong>de</strong>rivări succesive conduce la<br />
y ′ (x) = 1<br />
x z′ (ln x) = e −t z ′ (t)<br />
y(x) = z(ln x)<br />
y ′′ (x) = 1<br />
x 2 z ′′ (ln x) − 1<br />
x 2 z ′ (ln x) = e −2t ( z ′′ (t) − z ′ (t) )<br />
y ′′′ (x) = 1<br />
x 3 z ′′′ (ln x) − 3<br />
x 3 z ′′ (ln x) − 2<br />
x 3 z ′ (ln x) = e −3t ( z ′′′ (t) − 3z ′′ (t) + 2z ′ (t) )