If the coefficients A_{i} of the differential equation are real, then real-valued solutions ar

lunnatican4

lunnatican4

Answered question

2022-01-22

If the coefficients Ai of the differential equation are real, then real-valued solutions are generally preferable. Since non-real roots z then come in conjugate pairs, so do their corresponding basis functions xkezx, and the desired result is obtained by replacing each pair with their real-valued linear combinations Re(y) and Im(y), where y is one of the pair.

Answer & Explanation

autormtak0w

autormtak0w

Beginner2022-01-22Added 31 answers

Suppose the differential equation is y=y. Then teit and teit are linearly independent solutions. Suppose
f(t)=a1eit+a2eit
where a1 and a2 are complex numbers. Then f(t)=a1eit+a2eit=(a1+a2)cost+i(a1a2)sint=c1cost+c2sint.
The coefficients c1 and c2 are real if a1+a2 is real and a1a2 is a pure imaginary. That happens if and only if a1 and a2 are complex conjugates of each other.
We have c1=a1+a2,
c2=i(a1a2),
and a1=c1ic22,
a2=c1+ic22.
So every linear combination of eit and eit with complex coefficients is a linear combination of cost and sint with complex coefficients, and every linear combination of cost and sint with complex coefficients is a linear combination of eit and eit with complex coefficients.
Juan Spiller

Juan Spiller

Beginner2022-01-23Added 38 answers

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