Calculus 10th Edition

Published by Brooks Cole
ISBN 10: 1-28505-709-0
ISBN 13: 978-1-28505-709-5

Chapter 6 - Differential Equations - 6.1 Exercises - Page 403: 7


We can verify this by differentiating $y=-\cos x \ln|\sec x + \tan x|$ and putting $y$ and $y''$ into the differential equation $y''+4y'=2e^x$. This verifies the solution.

Work Step by Step

We will find the first and the second derivative of $y$: $y' = (-\cos x \ln|\sec x + \tan x|)' = -(\cos x)'\ln|\sec x + \tan x|-\cos x(\ln|\sec x + \tan x|)'$ where we used the product rule. Now we have $$(\cos x)' = -\sin x$$ from the table of derivative as well as $$(\ln|\sec x + \tan x|)'=\frac{1}{|\sec x+\tan x|}(|\sec x+ \tan x|)'$$ where we used the chain rule. Now $$(|\sec x+ \tan x|)'=\text{sign}(\sec x+ \tan x)(\sec x +\tan x)' =\text{sign}(\sec x+ \tan x) ((\sec x)' + (\tan x)') =\text{sign}(\sec x+ \tan x)\left(\frac{\sin x}{\cos^2 x} + \frac{1}{\cos^2 x}\right) = \text{sign}(\sec x+ \tan x) \frac{1+\sin x}{\cos^2 x}.$$ This helps us get $$(\ln|\sec x + \tan x|)' = \frac{\text{sign}(\sec x+ \tan x)}{|\sec x+ \tan x|}\times\frac{1+\sin x}{\cos^2 x}.$$ Since the sign function gives values $\pm 1$ depending of the "sign" of its argulent (whether it is positive or negative) we now that $\text{sign} x = 1/\text{sign} x$ because $1/\pm1=\pm 1$. Putting this into previous expression: $$(\ln|\sec x + \tan x|)' = \frac{\text{sign}(\sec x+ \tan x)}{|\sec x+ \tan x|}\times\frac{1+\sin x}{\cos^2 x} =\frac{1}{\text{sign}(\sec x+ \tan x)|\sec x+ \tan x|}\times\frac{1+\sin x}{\cos^2 x} = \frac{1}{\sec x+ \tan x}\frac{1+\sin x}{\cos ^2 x} = \frac{1+\sin x}{\cos^2 x \frac{1}{\cos x} + \cos^2 x\frac{\sin x}{\cos x}}=\frac{1+\sin x}{\cos x (1+\sin x)} = \frac{1}{\cos x}.$$ Assembling these results into the expression for $y'$ we get $$y' = \sin x \ln|\sec x +\tan x|-\cos x\frac{1}{\cos x} =\sin x \ln|\sec x +\tan x|-1. $$ Now let us find the second derivative: $$y''=(y')' = (\sin x \ln|\sec x +\tan x|-1)' = (\sin x \ln|\sec x +\tan x|)' - (1)'.$$ The derivative of a constrant $(1)'=0$ so we have $$y'' = (\sin x \ln|\sec x +\tan x|)'.$$ Using the product rule we get $$y''= (\sin x)'\ln|\sec x +\tan x|+\sin x (\ln|\sec x +\tan x|)'.$$ From the table of derivatives $$(\sin x)'=\cos x.$$ We already found $$(\ln|\sec x +\tan x|)' = \frac{1}{\cos x}$$ so finally we write $$y''=\cos x \ln|\sec x +\tan x| + \sin x\frac{1}{\cos x} =\cos x \ln|\sec x +\tan x| + \tan x.$$ Putting $y$ and $y''$ into the differential equation from the problem we have The Left side: $$y''+y=\cos x \ln|\sec x +\tan x| + \tan x + (-\cos x \ln|\sec x + \tan x|) = \tan x $$ The Right side is just $\tan x$. We see that the left side is equal to the right side so we can say that we verified that the given $y$ is a solution to the given differential equation.
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