Given the inhomogeneous first order differential equation below: <mrow class="MJX-TeXAtom-ORD">

Feinsn

Feinsn

Answered question

2022-06-14

Given the inhomogeneous first order differential equation below:
x ˙ ( t ) + a x ( t ) = q ( t )
where a R and q(t) is a continuous function.
It is given that the differential equation has the following 2 particular solutions:
x 1 ( t ) = e 4 t + 3 e 9 t x 2 ( t ) = e 4 t + 3 e 9 t
State below the value of the constant a and an expression for the function q(t).
When I apply the solution formula, I get 1 equation with 2 different unknowns. Can someone please help me?

Answer & Explanation

Stevinivm

Stevinivm

Beginner2022-06-15Added 18 answers

Because ± 1 both work as constants multiplying the e 4 t term, that must correspond to the homogeneous solution C e 4 t ,, which implies a=-4. (This is because arbitrary constants always arise from the homogeneous solution, never the particular solution.) The DE must now be
x ˙ ( t ) 4 x ( t ) = q ( t ) .
Next, we suppose that x ( t ) = e 4 t + 3 e 9 t ,, and compute x ˙ ( t ) 4 x ( t ) :
x ˙ ( t ) 4 x ( t ) = 4 e 4 t + 27 e 9 t 4 ( e 4 t + 3 e 9 t ) = 15 e 9 t = q ( t ) .

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