Undergraduate

Let y = cos ( kt ) . ( a ) Find the values of k such that y satisfies the differential equation 1 6

Let
y = cos(kt).
(a)
Find the values of k such that y satisfies the differential equation
16y=81y.
(Enter your answers as a comma-separated list.)
k =
(b)
For the values of k found in part (a), show that every member of the family of functions
y = A sin(kt)+ B cos(kt)
is also a solution of the differential equation.
We begin by calculating the following.
y = A sin(kt)+ B cos(kt) y= Ak cos(kt) Bk sin(kt) y=
Note that the given differential equation
16y=81y
is equivalent to
16y+81y =
.
Now, substituting the expressions for y and
y
above and simplifying, we have
LHS =16y+81y=16
+81(A sin(kt)+ B cos(kt))=16
16Bk2 cos(kt)+81A sin(kt)+81B cos(kt)=(8116k2)
+(8116k2) B cos(kt)=0
since for all value of k found above,
k2=
.

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