Solution of : d^4y/dx^4+8d^2y/dx^2+16=0

i am a bit confused about the ans. thanx.

y=(a+bx)cos2x+(c+dx)sin2x
this is easy do :
auxillary eqn=m^4+8m^2+16=0
(m^2+4)^4=0
thus m=-2i,2i,-2i,2i

Hope this helps :)

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