The given differential equation can be written as (D3−D)y=5t, where D≡dtd.
The auxiliary equation is, m3−m=0. Solving, we get m=0,1,−1.
The complementary function is, yc=c1+c2et+c3e−t=c1f1+c2f2+c3f3
The particular integral is yp=Pf1+Qf2+Rf3, where
P=∫W(f1,f2,f3)W1dx;Q=∫W(f1,f2,f3)W2dx; R=∫W(f1,f2,f3)W3dx;
and Wi is the determinant obtained from the Wronskian W(f1,f2,f3) by replacing the ith - column by the vector ⎝⎛00g(t)⎠⎞ where g(t)=5t.
Now,
W(f1,f2,f3)=∣∣100etetete−t−e−te−t∣∣=2 W1=∣∣005tetetete−t−e−te−t∣∣=−10t W2=∣∣100005te−t−e−te−t∣∣=5te−t W3=∣∣100etetet005t∣∣=5tet
Therefore,
P=∫−5t dt=−25t2 Q=25∫te−t dt=25(−te−t−e−t)=−25e−t(t+1) R=25∫tet dt=25(tet−et)=25et(t−1)
Hence,
yp=Pf1+Qf2+Rf3=−25t2−25e−t(t+1)⋅et+25et(t−1)⋅e−t=−25t2−25(t+1)+25(t−1)=−25t2−5
The general solution is
y=yc+ypy=c1+c2et+c3e−t−25t2−5=C1+c2et+c3e−t−25t2,where C1=c1−5.
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