Quadratic Solution to Homogeneous Second-Order ODEs

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SUMMARY

The discussion focuses on solving the homogeneous second-order ordinary differential equation (ODE) given by ax'' + bx' + cx = 0, specifically when the discriminant b² - 4ac = 0. The solution x = e^(kt) is confirmed, where k = -b/(2a). The user successfully substitutes the derivatives of x into the differential equation and simplifies it to demonstrate that the equation holds true, ultimately leading to a cancellation that results in zero.

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Homework Statement


Consider ax''+bx'+cx=0 for b^2-4ac=0 with k= -b/(2a).

Show x=e^(kt) is a solution.

The Attempt at a Solution



x=e^(kt)
x'=(-b/2a)e^((-b/2a)t)
x''=(-b^2/4a^2)e^((-b/2a)t)

Then i plugged these into ax''+bx'+cx=0 and simplified to get e^((-b/2a)t)(-((b^2)/2)+c)=0

I'm stuck at what to do next.

Thank you for any help.
 
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To make things easier, don't use the k=-b/2a substitution until you're ready to compute the final answer. This helps to keep it a lot more tidier. Just subsitute the derivatives of x into the DE, factor out e^kt and make use of k = -b/2a as well as b^2 - 4ac = 0. It'll all cancel out to give 0.
 
Ok, just got it. Thanks.
 
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