Verify every number of a family of functions is also a solution

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Homework Help Overview

The discussion revolves around a differential equation involving the function \( y = \cos(kt) \) and seeks to determine the values of \( k \) that satisfy the equation \( 4y'' = -25y \). Additionally, it explores whether the family of functions \( y = A \sin(kt) + B \cos(kt) \) also serves as solutions for those values of \( k \).

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the differentiation of \( y = \cos(kt) \) and the resulting expressions for \( y' \) and \( y'' \). There is uncertainty about how to verify that the family of functions satisfies the differential equation for the identified values of \( k \). Some participants question the interpretation of the problem's requirements regarding the verification process.

Discussion Status

The discussion is ongoing, with participants attempting to clarify the verification process for the family of functions. Some have provided expressions derived from the original function, while others emphasize the need to consider specific values of \( k \) as stated in the problem. There is no consensus yet on the verification method.

Contextual Notes

Participants are working under the constraints of the problem statement, which specifies the values of \( k \) and the requirement to verify the family of functions as solutions. There is some confusion regarding the correct interpretation of the verification task.

Aerospace93
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Homework Statement


(a)For what values of k does the function y=coskt satisfy the differential equation 4y''=-25y?
(b) For those values of k, verify that every number of the family of functions y=Asinkt+Bcoskt is also a solution.

The Attempt at a Solution


(a) y=coskt, y'=-ksinkt, y''=-k2coskt.
4(-k2coskt)=-25coskt; 4k2=25, so k=+/-5/2
(b) y= Asinkt+Bcoskt; y'= +/-Akcoskt+/-BKcoskt; y''= +-Ak2sinkt+/-bk2coskt
Im not sure how to verify this...
 
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Aerospace93 said:

Homework Statement


(a)For what values of k does the function y=coskt satisfy the differential equation 4y''=-25y?
(b) For those values of k, verify that every number of the family of functions y=Asinkt+Bcoskt is also a solution.

The Attempt at a Solution


(a) y=coskt, y'=-ksinkt, y''=-k2coskt.
4(-k2coskt)=-25coskt; 4k2=25, so k=+/-5/2
(b) y= Asinkt+Bcoskt; y'= +/-Akcoskt+/-BKcoskt; y''= +-Ak2sinkt+/-bk2coskt
Im not sure how to verify this...

Well, do you have 4y'' = -25y or not?
 
I believe you would if y''=-Ak2sinkt-Bk2coskt?
 
Did you miss where the problem says "for those values of k"? You are asked to show that y= A sin((2/5)t)+ B cos((2/5)t) and y= Asin((-2/5)t)+ Bcos((-2/5)t) both satisfy [itex]4d^2y/dx^2= -25y[/quote] for any A and B.[/itex]
 
ok so (for a positive k) y= Asinkt+Bcoskt; y'= -Akcoskt+BKsinkt; y''= -Ak2sinkt-bk2coskt
so,
-4[A(5/2)2sin(5/2)t-B(5/2)2cos(5/2)t= -25[Asin(5/2)t+Bcos(5/2)t
-25Asin(5/2)t-25Bcos/5/2)t= RHS
 
Last edited:

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